1ry multi-criterion design and 2d cosimulation model of 4

Multi-Criterion Design and 2D Cosimulation

Model of 4 kW PM Synchronous Generator

For Standalone Run-of-the-River Stations

Yucel Cetinceviz Mechatronics Dept.,

Kastamonu University

Kastamonu, Turkey

[email protected]

Durmus Uygun Electrical-Electronics Eng. Dept.,

Gediz University

Izmir, Turkey

[email protected]

Huseyin Demirel Electrical-Electronics Eng. Dept.,

Karabuk University

Karabuk, Turkey

[email protected]

Abstract— This study reports the analytical computation

including performance characteristics such like load line voltage

and output power in combination with coupled-field circuit

analysis of a 4kW direct drive permanent magnet synchronous

generator (PMSG) to be used in a micro-scale run-of-the-river-

station application. The specifications such like slot opening, pole

embrace and magnet length of PMSG are optimized by using

parametric approach including multi-criterion design

optimization. Based on the optimized design, the model has been

exposed to some transient coupled-field circuit analyses based on

variable river flow speed and variable ohmic load conditions. The

analytical studies related to finite element methods and

conducted parametric approaches verified the effectiveness of the

employed dynamic co-simulations.

Keywords—run-of-the-river-stations; coupled-circuit analysis;

PM generator; multi-criterion design optimization; parametric

approach

I. INTRODUCTION

The need for the energy is increasing day by day along with rapid population growth and industrialization. The fossil fuels such as coal, oil and natural gas which are used to meet these requirements are being replaced by renewable energy sources like sun, wind, geothermal, hydraulic and ocean resources [1, 2]. Energy cycle plants are obtained with the turbines placed in natural energy sources such like a river, wave or tidal areas which are using the power of liquid flow [3-5]. Compared to other renewable energy technologies, hydrokinetic systems are low-cost energy conversion systems due to their low investment expenditures and maintenance fees [6]. Accordingly, micro-scale run-of-the-river-stations are offering cost-effective solutions especially in rural areas [6, 7]. As long as the rivers and creeks don’t dry up, they offer the advantage of being a constant source [8]. Besides, these river plants relatively have higher kinetic-energy densities when water flow velocity exceeds 2 m/s and thus increase potential commercial investments [9].

In such kinds of power plants; commonly variable speed permanent magnet synchronous generators (PMSGs) [5, 6 and 8-13] and doubly-fed induction generators (DFIGs) [2] are employed. Variable-speed turbine structure housing PM generator offers appropriate solutions to meet the energy needs

of the rural areas.

The objective of this article is related to present the design and dynamic performance analysis of a 4kW PMSG in order to get maximum available energy from water flow. To gain maximum available energy, detailed design and analysis was carried out to cover the operational aspects such as start-up torque, cogging torque, ripple, flow interactions, efficiency, rated power and terminal voltage. The cogging torque is a common issue for permanent magnet machines [11]. The torque ripple in the cogging torque is related to the harmonics in the back-EMF. To reduce it, the effect of the slot opening, the magnet thickness and the pole arc/magnet arc ratio (embrace) of the cogging the torque were investigated parametrically. Thus, this study gives an opportunity analysis of dynamic performance of 4 kW PMSG supplying different loads and variable generator speeds related to water flow by using coupled two dimensional (2D) electromagnetic field -circuit model.

II. DESIGN SPECIFICATIONS

A. Background of the Design

Prior to preliminary prototyping of a custom electrical machine, the design can be initiated by defining basic parameters such as machine type (synchronous, asynchronous, DC, reluctance machine, etc.), structure, nominal power, rated speed, number of pole pairs and rated voltage and by determining additional characteristics like efficiency, cost and manufacturability [14]. A real machine design starts with the selection of main dimensions of the machine. These main dimensions indicate the air gap diameter (Ds) measured via stator slot tips and stack length (L) of the machine. In electrical machine design, there are some empirical definitions in the variation interval of current density and flux density parameters including the selection of magnetic loading values.

The machine constant (C) of a well-designed power conversion system is the basic element in construction. The machine constant jointly expresses apparent power Si or active power Pi presented with given rotor volume. Another factor affecting the power of the machine is Dis

2.L multiplication [15-

17]. So, the problem is to separate these two parameters as air gap diameter (Dis) and equivalent stack length (L). In the

This work is funded by The Scientific and Technological Research Council of Turkey (TUBITAK) under grant numbers 113E782 and 113E577.

4th International Conference on Renewable Energy Research and Applications Palermo, Italy, 22-25 Nov 2015

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literature, various ratios like stack length/pole arc [15, 18 and 19] and stack length/air-gap diameter (or stator inner diameter) have been used to separate these parameters. In other study, the stack length/air-gap diameter ratio has been used as L=Dis/4. Additionally, stator outer diameter/stator inner diameter ratio is an important parameter to declare and meet temperature effects, loss and efficiency requirements. Various values are indicated for that ratio in the literature as well. While it is given as 1.24-1.26 for the machines with high number of poles in [18], it is approximately indicated as 1.66 in [14] and as √3 in [20]. The generator taken into consideration will have 24 poles due to high energy harvesting requirements even in low shaft speeds and it is not clear which value or interval is appropriate for this application.

Owing to this complex processes mentioned above, the necessity is increasing for the verification of the design and optimum solution delivery. This also leads the process to be more complex, expensive and time consuming. Therefore, there is a need for simulation and analysis model to address this complexity [21]. So, in this study, an integrated field-circuit model was used for analysis of the dynamic performance of the machine combined with electromagnetic field solution.

B. Analytical Modelling Study

In many direct drive application, high torque and low speed

are required. Thus, a multiple pole, inner rotor-structured

machine topology has been considered by using NdFeB

magnets with high magnetic flux density. Besides, another

reason for choosing an inner-runner structure is shorter

manufacturing process and easy mounting of the generator.

This configuration is also suitable for cooling since the

windings where copper losses (heat) are taking place are

formed around the rotor. In this section, analytical calculations

for the design of the machine according to the specifications

given in Table II are performed.

TABLE I. GIVEN TECHNICAL DATA FOR THE GENERATOR

Parameter Data

Rated Output Power (kW) 4

Rated speed (rpm) 250

Frequency (Hz) 50

Number of phases 3

Rated Voltage (V) 400

Rated Power Factor 0.95

Target Efficiency >= %92

If the design of the generator is initiated with machine

constant, the following equation can be derived;

2

0 1 1 2

1

60 gap

f i w g

is

SC K K A B

D Ln (1)

where Kf is the form factor, αi is the magnetic flux density

form factor based on magnetic saturation on stator tooth, Kw1

is the winding factor, A1 is the specific electrical loading and

Bg is the magnetic loading values.

As insisted before, Dis which is the air-gap diaemeter (or

stator inner diameter in other words) is one of the most

important parameter of both inner and outer runner PM

generators and can be obtained as;

2 2

3 32is is

is

D L p D LD

x (2)

But, these two equations are not enough to give a

comprehensive volume expression of the machine. So, stator

outer diameter must be taken into consideration in terms of

slot dimension as follows [22];

2( )o is s csD D h h (3)

where Do is the outer diameter of the generator, hs is the

slot depth and hcs is the stator core height. These two

parameters can been calculated as [17, 18];

1 16 1n

s

g giscon fill con fill

ts ts

W I Ah

B BDj K j K

B B

(4)

12 2 2

gi is

cs

cs cs

BDh

LB p B

(5)

Additionally; the following ratio can be given to declare

and meet temperature effects, loss and efficiency

requirements;

1

21 1

2

aspect g go

is s ts cs

K B BD i

D N B p B

(6)

where Kaspect is the limitation factor. The ratio of stack

length to air gap diameter is another significant parameter

which is also known as L/D [23];

12; 0.6 3.0

p

is

LL

D

(7)

With respect to all parameters calculated by using

analytical method, the initial design parameters of the

generator can be demonstrated in Table II.

TABLE II. COMPUTED OUTPUT DATA FOR 4KW GENERATOR

Parameter Data

Load Line Voltage 405 V

RMS Line Current 6.49 A

Specific Electrical Loading 28699.9 A/m

Armature Current Density 3.94 A/mm2

Iron Core Loss 36.56 W

Armature Copper Loss 338.52 W

Total Loss 375.08 W


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Output Power 4539 W

Input Power 4914 W

Efficiency (%) 92. 36

Power Factor 0.9851

Rated Torque 188.707 Nm

Short Circuit Current 27.38 A

As a result of analytical calculations, the generator model which is consisting of calculated analytical dimensions can be illustrated as seen Fig.1.

Fig. 1. Objective 4kW PM generator for run-of-the-river station application.

C. Multi-Criterion Parametric Optimization

Parametric analysis is employed in the case of optimizing the model that is preconfigured via theoretical computations in electric machine design process. In design process, there are many software and modules realizing simulations in order to shorten the development period prior to construction of the real model. Due to comprising numerous sub-modules, ANSYS RMxprt software is used in detailed parametric analyses.

Optimization problem is to find an acceptable design which fulfils all the requirements. However, there are many possibilities for selecting an objective function. Due to the fact that a PMSG is including many parameters, it is useful to use a multi criterion optimization approach of which main idea is presented in the following equation;

1 1 1 1

2 2 1 2

1

( , , )

( , , ): : :

( , , )

n

n

n m n m

x f x x Q

x f x x QX Q

x f x x Q

(19)

Where x1 and x2 are two feature vectors and Q is the evaluation criteria. Besides, the optimization based on the analytical model which is implemented in order to obtain the optimal geometric structure and field distributions of PMSG is also preferred to minimize total losses at nominal operation

conditions. In that case some parameters like outer diameter of the stator and rotor, stack length of PMSG, slot opening and magnet thickness which are forming the basic of the generator are chosen to be optimized of which computation way is presented below;

0

min max 0

, , ,

, , , ,

( )

loss o s mag

o s mag

min P D L B t

Functions x x x x D L B t

f x

(20)

In the scope of the above mentioned explanations, slot

opening, magnet thickness and the pole embrace parameters have been optimized within acceptable ranges.

Slot opening must be in the size of the conductor. In the case of using very small slot opening, it may be possible to see leakage flux from one tooth to another. Therefore, the length of the slot opening must be higher than air gap length. On the other hand; for wider slot opening values, cogging torque parameter increases. In accordance with all these limits, it is necessary to determine an optimum slot opening value. In the literature, it is stated to choose a slot opening value between 2 and 3 mm [18, 24]. So, the value of slot opening has been recalculated for better efficiency and output voltage level and lowest cogging torque as a result of multi-criterion parametric analyses between 1mm and 3mm length with a sensitivity of 0.1mm. And the derived results are illustrated in Fig.2. It can be easily seen on the graph that especially cogging torque is very close to zero for the selected slot opening value (optimum region).

Optimum

region

Fig. 2. Effect of variable slot opening on load line voltage, output power,

efficiency and cogging torque parameter of generator.

In direct drive generators, the effect of cogging torque is considerably significant since the designed generators may be used in very low speed application. One of the methods to eliminate cogging torque is “skewing of either stator slots or magnets” in which we preferred magnet skewing owing to easy manufacturing cases. Another way to decrease cogging torque and make some improvements in parameters like output power and load line voltage is to vary magnet arc/pole arc ratio (embrace) within an acceptable range. So, embrace has been parametrically solved between 0.5 and 1 with 0.02 steps and the effect of that variation on load line voltage, efficiency, output power and cogging torque is illustrated in Fig.3.


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Optimum

region

Fig. 3. Effect of variable pole arc/magnet arc ratio (embrace) on load line

voltage, output power, efficiency and cogging torque parameter of generator.

Nowadays, the magnets with high energy density are generally used in electrical machine applications. N40SH type NdFeB magnets will be preferred in this generator design due to their high energy density and effective operational functions in applications with high temperature. The magnetic flux density (Bm) of these magnets is around 1.23T. One of the most significant factors in selecting magnet type is the thickness of the magnet not allowing demagnetization in a short time [22]. But on the other hand, thickness is a factor which is increasing the cost of the machine.

Since cost/efficiency ratio is to be considered as a design parameter, a reasonable magnet thickness should be chosen. A parametric study between 3mm and 10mm thickness with 0.2mm steps has been achieved and the effect of that variation on load line voltage, efficiency, output power and cogging torque is illustrated in Fig.4.

Fig. 4. Effect of variable magnet thickness on load line voltage, output

power, efficiency and cogging torque parameter of generator.

III. ELECTROMAGNETIC FIELD AND COUPLED CIRCUIT

ANALYSES

In this section, the dynamic design results of 4kW PMSG which will be manufactured in the project have been presented by using the model derived from optimization studies. The main idea behind executing a coupled electromagnetic analysis is that the results obtained via simulations will provide comprehensive information and data related to the behaviour of the machine in different conditions. Moreover, it is possible to confirm the accuracy and precision of analytical calculations.

Related model given in Fig.5 is used to carry out simulations in two steps. Primarily; the generator was operated for a time period of 100ms at varying speed rates ranging from 100rpm to 450 rpm for constant load condition (Rload is 44Ω as a result of analytical calculations).

RIVER STATION

PMSG

General arguments

for any application:

Size / Weight

Efficiency

Torque

Speed

Cost

Manufacturability

Power Density

Generator speed regime

related to water flow speedTransient

Analysis

using

FEA Variable load

Rectifier Block Inverter Block2D Electromagnetic Model of

PMSGLoad

CO-SIMULATION RESULTS

Fig. 5. Design methodology for complete co-simulation process.


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The dynamic performance of the PMGS relating to consant load and varying speed conditions have been summarized in Fig.6.

Fig. 6. Load line voltage and generator output power parameters for varying

generator speeds and constant load.

For varying generator shaft speeds (herein simulated as variable water flow speed), it can be clearly resulted that load line voltage and output power of the generator are curvaceously increasing as long as the shaft speed of the generator steps up.

For another study with Rload is 36Ω in which generator

operation regime with variable speed is shown in Fig.7,

transient coupled circuit field results demonstrated in Fig.8

have been obtained for a 200ms time period through 100µs

time steps. In the case that this time step is decreased which is

also leading more complex and longer simulation time, better

results can be derived.

Fig. 7. Generator speed regime related to water flow speed.

(a)

(b)

Fig. 8. Simulation results derived from variable speed operation, a) Generator side measurements b) Load side measurements


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Besides, the output characteristic of the machine related to simulation results realized separately for different load conditions (22-26-30-36-40-44-50-56Ω) and rated speed (250rpm) has been derived as seen in Fig.9.

Fig. 9. Load line voltage and generator output power parameters for varying

load conditions at 250rpm rated speed.

Different from the analyses derived before, the output characteristic of the machine has been pointed by defining an operation cycle related to time in which variable load conditions have been saved as 44-28-8-22Ω and illustrated in Fig.10 at rated speed and the results have been shown in transient variation in Fig.11 for both generator and load sides, respectively.

Fig. 10. Operating variable load(ohmic).

(a)

(b)

Fig. 11. Simulation results derived from variable load and constant flow operation, a) Generator side measurements b) Load side measurements.


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It can be noted that the reaction of the generator with in

predefined operation cycle is quite good but especially at the

points where value of the load is increased unexpectedly and

instantly, some ripples are observed on generator phase and

line voltage due to instant torque variation. On the other hand,

the observation on load side measurements indicates that the

generator is quite powerful to supply appropriate line voltage

for the load at the moments where the load is varied.

IV. CONCLUSION AND FUTURE STUDIES

In this paper, the design of a 4kW permanent magnet synchronous generator suitable for standalone or off-grid run-of-the-river-station turbine application has been developed. The results of analytical PMSG design and concerned optimization methods are reported in the paper. Thereto, the effectiveness of the proposed electrical machine configuration in terms of output power, efficiency, cogging torque and load line voltage have been demonstrated via 2D dynamic transient co-analysis approach by indicating compliance of simulated and real load conditions prior to the fabrication of the designed and optimized machine. But, the study on 380V, 4kW generator design with in-runner configuration is not yet complete. As a part of project; the testing, verification and advanced analysis studies need to be performed posterior to completion of prototyping process. Next to this, future studies on optimized machine are going to focus on eddy current loss minimization in permanent magnets as well as copper loss optimization in stator windings.

ACKNOWLEDGMENT

This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under grant numbers EEEAG-113E782 and partially EEEAG-113E577 since similar design methods and approaches have been utilized. The authors would like to thank TUBITAK for their financial supports.

REFERENCES

[1] Liang, Q., Yan, X., Liao, X., Cao, S., Zheng, X., Si, H., ... & Zhang, Y. “Multi-unit hydroelectric generator based on contact electrification and its service behavior”, Nano Energy, 16, 329–338, (2015).

[2] Camara, M. B., Dakyo, B., Nichita, C., & Barakat, G. “Simulation of a Doubly-Fed Induction Generator with hydro turbine for electrical energy production”, Joint Symposium on Advanced Electromechanical Motion Systems & Electric Drives, 1-6, (2009).

[3] Khalid, S. S., Liang, Z., & Shah, N. “Harnessing tidal energy using vertical axis tidal turbine”, Research Journal of Applied Sciences, Engineering and Technology, 5(1), 239-252, (2012).

[4] Zeiner-Gundersen, D. H. “A novel flexible foil vertical axis turbine for river, ocean, and tidal applications”, Applied Energy, 151, 60-66, (2015).

[5] Khan, M. J., Iqbal, M. T., & Quaicoe, J. E. “Tow tank testing and performance evaluation of a permanent magnet generator based small vertical axis hydrokinetic turbine”, Power Symposium, 1-7, (2008).

[6] Davila-Vilchis, J. M., & Mishra, R. S. “Performance of a hydrokinetic energy system using an axial-flux permanent magnet generator”, Energy, 65, 631-638, (2014).

[7] Anagnostopoulos, J. S., & Papantonis, D. E. “Optimal sizing of a run-of-river small hydropower plant”, Energy Conversion and Management, 48(10), 2663-2670, (2007).

[8] Rohmer, J., Sturtzer, G., Knittel, D., Flieller, D., & Renaud, J. “Dynamic model of small hydro plant using archimedes screw”, 2015 IEEE International Conference on Industrial Technology (ICIT), 987-992, (2015).

[9] Birjandi, A. H., Woods, J., & Bibeau, E. L. “Investigation of macro-turbulent flow structures interaction with a vertical hydrokinetic river turbine”, Renewable Energy, 48, 183-192, (2012).

[10] Navasardian, A. A. “Underwater hydro unit with the combined energy extraction”, 2014 International Conference on Renewable Energy Research and Application (ICRERA), 158-161, (2014).

[11] Khan, M. J., Iqbal, M. T., & Quaicoe, J. E. “Dynamics of a vertical axis hydrokinetic energy conversion system with a rectifier coupled multi-pole permanent magnet generator”, IET renewable power generation, 4(2), 116-127, (2010).

[12] Neely, J. C., Ruehl, K. M., Jepsen, R., Roberts, J. D., Glover, S. F., White, F. E., & Horry, M. L. “Electromechanical emulation of hydrokinetic generators for renewable energy research”, In Oceans-San Diego, 1-8, (2013).

[13] Khan, M. J., Iqbal, M. T., & Quaicoe, J. E. “Effects of Efficiency Nonlinearity on the Overall Power Extraction: A Case Study of Hydrokinetic-Energy-Conversion Systems”, IEEE Transactions on Energy Conversion, 26(3), 911-922, (2011).

[14] Pyrhonen, J., Jokinen, T., Hrabovcova, V., “Design Of Rotating Electrical Machines”, John Wiley & Sons, Ltd,(2008).

[15] Mihai, A.M., Benelghali, S., Simion, A., Outbib, R., Livadaru, L. “Design and FEM analysis of five-phase permanent magnet generators for gearless small-scale wind turbines”, International Conference on Electrical Machines, 150 – 156, (2012).

[16] Tapia, Juan A., et al. “Optimal design of large permanent magnet synchronous generators”, IEEE Transactions on Magnetics, 49.1 (2013): 642-650.

[17] Huang, Surong, et al. “A general approach to sizing and power density equations for comparison of electrical machines.”, IEEE Transactions on Industry Applications, 34.1 (1998): 92-97.

[18] Boldea, Ion, and Syed A. Nasar. The induction machine handbook. CRC press, 2001.

[19] Murthy, K. M. Computer-aided design of electrical machines. BS Publications, 2008.

[20] GIERAS, Jacek F.; WANG, Rong-Jie; KAMPER, Maarten J. Axial flux permanent magnet brushless machines. New York, NY: Springer, 2008.

[21] Makolo, P., Wind Generator Co-Simulation with Fault Case Analysis, (Master of Science Thesis), Chalmers University of Technology, Department of Energy and Environment, (2013).

[22] Damiano, A., Marongiu, I., Monni, A., & Porru, M. Design of a 10 MW multi-phase PM synchronous generator for direct-drive wind turbines in Industrial Electronics Society, IECON 2013-39th Annual Conference of the IEEE, (2013, November), pp. 5266-5270.

[23] Faiz, J., Ebrahimi, B. M., Rajabi-Sebdani, M., & Khan, M. A. “Optimal design of permanent magnet synchronous generator for wind energy conversion considering annual energy input and magnet volume”, IEEE International Conference on Sustainable Power Generation and Supply, 2009. SUPERGEN'09, (pp. 1-6).

[24] Gieras, Jacek F. Permanent magnet motor technology: Design and Applications. CRC press, 2002.


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1ry multi-criterion design and 2d cosimulation model of 4

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