TheOptimized Design of NPC Three-Level Inverter Forced-Air Cooling System…1

http://dx.doi.org/10.6113/JPE.2012.12.4.000 JPE XX-X-XX

The Optimized Design of NPC Three-Level Inverter Forced-Air Cooling System Based on Dynamic

PowerLoss Calculation of Maximum Power-Loss Range

Shi-Zhou Xu*, Feng-You He†

†*Dept. of Information and Electrical Eng., China University of Mining and Technology, Xuzhou, China

Abstract

In some special occasions with strict size requirements, such as mine hoist, how to improve the design accuracy of forced-air

cooling system of NPC three-level inverter is a key technology to improve the power density and decrease the volume. A fast power-loss calculation method was brought in firstly, with its calculation principle introduced in detail, and meanwhile the computation formulas were deduced. Secondly, the average and dynamic power losses of a 1MW mine hoist acting as the research target were analyzed, and the forced-air cooling system model based on a series of theoretical analyses was designed with the average power loss as heat source. The simulation analyses proves the accuracy and effectiveness of this cooling system during unit lifting period. Finally, according to the analysis of the periodic working condition, the maximum power-loss range of NPC three-level inverter under multicycle operation was obtained and its dynamic power loss was taken into the optimized cooling system model as the heat source to solve the power device damage caused by instantaneous heat accumulation. The effectiveness and feasibility of optimization design based on the dynamic power loss calculation of the maximum power-loss range was proved by simulations and experiments.

Keywords:Cooling system, power loss calculation, NPC three-level inverter, heatsink optimization

I. INTRODUCTION In recent years, the applications of NPC three-level

inverter increases gradually with the increasing capacity of mine hoists. As a special application, the mine hoist working conditions are particular and complex with limited space, which requires for a much higher power density of the inverter. For the entire inverter system, the cooling system occupies a large space and its size plays an important role in improving the power density. So, The research object of this article is the forced-air cooling system of three-level inverter, and the research goal is to optimize the cooling system into an effective and accurate one with maximum power density

and minimum volume approximately, based on the maximum

power loss calculation of interval dynamic loss. As shown in [1], about 60% inverter failures are caused by

high temperature, and a double failure rate comes out with every 10℃ rising. Therefore, on the basis of an accurate power loss calculation of power devices in inverter, it will be possible and necessary to design an efficient cooling system to improve the thermal stability of the whole system. However, if an accurate analysis and design of the cooling system want to be obtained, the premise is to calculate the inverter power loss accurately, and until now, some research has been done by some experts and scholars. Currently, there are many researches on power-loss calculation and thermal analysis for single IGBT module and two-level inverters[2]-[6]. That[7]-[10] did not consider the impact of junction temperature of power devices to power losses, is the main reason causing errors between their theoretical calculations and experimental results, where Dieckerhoff [10] considered the switching power loss of power device has a linear relationship with its withstanding voltage, while this

†CorrespondingAuthor:xushizhousiee@163.com Tel: +86-15062190287, Fax: +86-0516-80139933, China Univ. ofMining and Tech. *Dept. of Information and Electrical Eng., China Univ. of Mining and Tech.,China

2Journal of Power Electronics, Vol. 12, No. 4, July 2012

assumption is approximately valid only in 20%± range of the test voltage. A much accurate losses calculation and heat dissipation method was introduced in [11], but it didn’t take all the heat sources in consideration, which has an effect on the power devices and thermal analysis. In [12], the transient modeling of loss and thermal dynamics in power semiconductor devices is analyzed, while it needs improve the model by considering the peripheral circuits. In [13], it has been proposed that the power losses of inverters can be calculated by simulating the model of power device. However, the model is built on the basis of the actual operation conditions and analysis of each power device condition rule. Some conducting loss calculation methods for IGBTs and diodes of inverters were stated in [13] and [14], and all of them were completed by using on-state voltage drop, current and duty cycle to calculate. For switching losses, a much more direct way is that by a large number of repeated trials, a great deal of test data can be collected, and then the approximate data can be obtained based on various effecting factors through curve fitting. Even though it's an easy way to calculate, the results can be affected by different experimental conditions easily. A much more accurate method is cutting the switching process of power devices into several stages and calculating each stage's power loss by integral[9],[15].However, the value of this calculation method is the average power loss of inverter, which is not suitable for the hoist with a very large overload coefficient at different sections of the whole operation cycle. In [16], Xiang-ning He proposed an approximate on-line model of inverter power loss based on IGBT off-line test platform, with which the commutation mode, modulation method and load type of inverters can be equaled. On the basis of this equal, the total power losses at different junction temperatures can be obtained, and after the interpolation calculation, the power loss at a certain temperature will come out. Even though this method can get a more accurate power loss under certain temperature, the result is an average total power loss under this temperature and cannot meet the periodic load with a large instantaneous overcurrent. From what has been stated above, all the former methods are only valid for those inverters under normal working conditions. For high power mine hoists, they have very large overload currents in the acceleration and deceleration periods in short time, and the instantaneous total power loss within these ranges is very large too. This range composed by the deceleration period of former cycle and acceleration period of latter period is the so called the maximum power range. But during the constant velocity range, the power loss is much less and can spread evenly in a much longer period. Even though the huge heat generated during this range is weakened for the inverter working in only one cycle or many cycles with long intervals, and can not affect the inverter under this condition, for those inverters under multicycle operation with

no intervals or only a little interval, it will be a lethal damage. When using the methods above to calculate the total power loss in one cycle as a heat source to design the air-forced cooling system, the heat accumulation phenomenon caused by large power loss in the maximum power range will be covered by the small average value with a long cooling period, which covers the actual capacity of the cooling system and will cause serious damage for mine hoists under special operating conditions. Therefore, in this paper, according to the mine hoist operating condition, the maximum power range was extracted to calculate the dynamic power loss of NPC three-level inverter as the heat source to optimize the design of the forced-air cooling system.

At present, many studies have been done on forced-air cooling system design in inverters, and the axial flow induced draft fan is one of the most common styles as a research object. In accordance with the installation position, this style can be divided into overhead and knapsack types, and both of them have their own advantages in space saving. The research object of this paper is overhead cooling, whose structure is shown in Fig.1. It is indicatedin [17] and [18]that empirical formulas and analysis model have been an alternative method to describe many models accurately, and the authors have discussed the theory power limit of converter system to optimize the heat sink. A fin type array of forced convection cooling plate was described in [19]. A practical guidance of selecting heat sink was given in [20], and some design processes were recommended as well. Leonard et al. summarized the bypass flow characteristics in plate-fin heatsinks and put forward their design model to calculate the bypass flow [21]. For the purpose of calculating bypass flow in accuracy, Hossain et al. in [22] proposed a comprehensive analysis process and deduced a simple empirical equation, which can get enough analysis and design precision. In this paper, the main work is to use the power loss calculated before as heat source to build the accurate model of forced-air cooling system and analyze the cooling capacity, which determines the thermal capacity of the fan, and then optimize the whole system.

TheOptimized Design of NPC Three-Level Inverter Forced-Air Cooling System…3

Front View

View45 Lateral view

Fan

Duct

Heatsink

dW

S

a

mW IGBT

DW

DH

Air flow inletNegative pressure chamber

Air flow outlet

Fig.1. Forced-air cooling system structure of NPC three-level inverter

Fast power-loss calculation

Average power loss

Cooling system design and analysis

Cooling system optimization based on maximum power range

Experiment verification

Simulation and experiment

Fig.2.Flow chart of cooling system optimization based on maximum power-loss range

The flow chart is shown in Fig.2. This paper proposed a fast power-loss calculation method firstly to calculate the total average power loss of NPC three-level inverter in Section II. The cooling system was analyzed and designed using average power loss as heat source in Section III. Then the experiment proved the disadvantages of the average power loss acting as heat source and meanwhile using the maximum power range based on the dynamic power loss calculation of NPC three-level inverter under dynamic conditions to optimize the design in Section IV. Finally, the simulation and experiment verifies the feasibility and

effectiveness of the proposed optimization.

II. DYNAMIC POWER LOSS CALCULATION OF NPC THREE-LEVEL

INVERTERS

The topology structure of back-to-back NPC three-level converter is shown in Fig. 3, which can be divided into NPC-rectifier and NPC-inverter parts and the main research object of this paper is the latter one.

+

-

DC Link

O

NPC-InverterNPC-Rectifier

P

N

Grid Load

T1

T2

T3

T4

D5

D6

D1

D2

D3

D4

A

Fig.3. The topology of back-to-back NPC three-level converter

The power devices used in this topology are IGBTs and

diodes, and the power loss of IGBT TP includes conduction

loss cond TP − and switching loss _sw TP .

T _cond T sw TP P P−= + (1)

The power loss of fast recovery diode DP consists of

conduction loss cond DP − and switching loss _sw DP , namely

_D cond D sw DP P P−= + . (2)

(1)Conduction loss Due to the existence of initial saturation voltage drop and

conduction resistance, power devices will generate conduction losses in the process of conduction. Meanwhile, both initial saturation voltage drop and conduction resistance will change with temperature linearly, and for the IGBT with fast recovery diode, its conduction properties can be described approximately in the following linear formula respectively.

_ _ _25

_ __ 25

[ ( 25 )]

[ ( 25 )]CE ce r T j T

ce V T j T

v r K T i

V K T

= −

+ + −℃

℃

+ ℃

℃(3)

_ __ 25

_ 25

[ ( 25 )]

[ ( 25 )]F F r D j D

F V D j D

v r K T i

V K T− −

= −

+ + −℃

℃

+ ℃

℃ (4)

In (3) and (4), CEv and Fv stand for the actual voltage

drops of IGBT and fast recovery diode respectively; _j TT

and _j DT are actual junction temperatures of IGBT and fast

4Journal of Power Electronics, Vol. 12, No. 4, July 2012

recovery diode respectively; _r TK is the temperature

coefficient of temperature impacting on conduction resistance

of IGBT. _r DK is the temperature coefficient of temperature

impacting on conduction resistance of fast recovery diode.;

_V TK is the temperature coefficient of temperature

impacting on voltage drop of IGBT; V DK − is the temperature

coefficient of temperature impacting on voltage drop of fast recovery diode; i is the output current of inverter.

The fundamental waves of output AC voltage of inverter using PWM can be expressed as follow

2 cosoutu U θ= (5)

where, outU is the root mean square of actual voltage and

θ is the phase angle. When switching frequency is high enough, the output

current i can be approximately equivalent to sinusoidal current.

2 cos( )outi I θ ϕ= − .(6)

In (6), outI is the root mean square of actual current;ϕ is

the phase angle between actual current and actual voltage. Duty cycle of the given PWM method is as follow

1 cos2

M θξ += .(7)

In (7), M is the modulation of PWM (peak value of phase voltage divided by 1/2 DC voltage of bridge arm). Under constant frequency, the duty cycle can be simplified as the function of phase angleθ .

In accordance with (3) and (4), the conduction power losses of IGBT and fast recovery diode with a sinusoidal output current can be deduced respectively as follows.

_ 25

2_

_ _25

1 cos2( )[2 8

1 cos( 25 )] 2( )8 3

[ ( 25

cond T out ce

V T j out

ce r T j

MP I V

MK T I

r K T

ϕπ

ϕπ

− = +

+ − + +

× −

℃

℃

℃

+ ℃)]

(8)

_ 25

2_

_ _25

1 cos2( )[2 8

1 cos( 25 )] 2( )8 3

[ ( 25

cond D out F

V D j out

F r D j

MP I V

MK T I

r K T

ϕπ

ϕπ

− = −

+ − + −

× −

℃

℃

℃

+ ℃)]

(9)

In (8) and (9), cond TP − and cond DP − are conduction

power losses of IGBT and fast recovery diode respectively. (2)Switching loss The switching loss comes from the process of switching on

and off, and under special test conditions, the power losses of switching on and switching off can be obtained indirectly by integrating the product of voltage and current to time, during which the differences between actual current/voltage and reference current/voltage must take into consideration. In one switching cycle, the switching losses of IGBT and fast recovery diode can be represented respectively as

_ _

_ _

2( )

[1 (125 )]

outsw T s on off swT I

rated

ccswT V swT T j T

rated

IP f E E KI

V K K TV

π

= +

× × +

_℃-

(10)

_ _

_ _

2

[1 (125 )]

out ccsw D s sp swD I

rated rated

swD V swD T j

I VP f E KI V

K K Tπ

=

× × + _D℃-

(11)

In (10) and (11), sf is the carrier frequency; onE is the

single pulse switching-on loss of IGBT under rated

conditions; offE is the single pulse switching-off loss of

IGBT under rated conditions; spE is the single pulse

switching-off loss of fast recovery diode under rated

conditions; ccV is bridge arm voltage; ratedI and ratedV are

reference current and reference voltage respectively;

_swT IK is the current coefficient of current amplitude

affecting the switching loss of IGBT; _swT VK is the voltage

coefficient of bridge arm voltage affecting the switching loss

of IGBT; _swD IK is the current coefficient of current

amplitude affecting the switching loss of fast recovery diode;

_swD VK is the voltage coefficient of bridge arm voltage

affecting the switching loss of fast recovery diode; _swT TKis the temperature coefficient of temperature affecting the

switching loss of IGBT; _swD TK is the temperature

coefficient of temperature affecting the switching loss of fast

recovery diode. There are certain differences among onE ,

offE and spE in single pulse switching loss, which should

choose reasonably. Generally speaking, the curve of switching loss changing

with load current, on the basis of the test voltage at 125℃junction temperature, can be found in IGBT parameter datasheet.When the power devices operate with different voltages and junction temperatures, their power losses can be modified by the power function of ratio between actual withstanding voltage and test voltage as well as the power function of ratio between actual junction temperature and test

TheOptimized Design of NPC Three-Level Inverter Forced-Air Cooling System…5

junction temperature. Therefore, the IGBT switching on and

off loss sw,T ( )E I of single pulse power can be expressed as

sw,T sw,T

sw,T on,T off,T

2sw,T sw,T sw,T

vj,Tce

base base

( ) ( ) ( )

( )D K

E I E I E I

A I B I C

TUU T

= +

= + +

×

(12)

In (12), sw,TA , sw,TB and sw,TC are quadratic curve

fitting coefficients of switching loss changing with current

under test conditions; sw,TD is the correction coefficient of

test voltage baseU ; sw,TK is the correction coefficient of

test junction temperature baseT (under normal circumstances is

125 ℃); ceU is the actual withstanding voltage.

The switching on loss of fast recovery diodes is very small, which can be ignored, and only the reverse recovery loss

spE during switching off should take into account. Therefore,

the single pulse switching loss of fast recovery diodes is

equal to the reverse recovery loss spE approximately, and like

IGBT, according to the parameter datasheet, the switching loss can be expressed as

rec,D rec,D

2rec,D rec,D rec,D rec,D

vj,Dce

base base

( ) ( )D K

E I A I B I C

TUU T

= + +

×

(13)

In (13), Arec,D, Brec,D and Crec,D are quadratic curve fitting coefficients of switching loss changing with current under test conditions; Drec,Dis the correction coefficient of test voltageUbase; K rec,D is the correction coefficient of test junction temperate Tbase.

In view of the symmetry of both bridges in system and upper and lower arms in one bridge, the total power loss

TPP is equal to the sum of six half bridges, and can be

defined as follow using the half bridge of phase A.

6 ( _ _ )TP T a D aP P P= × + (14)

Therefore, the following section will only calculate the power loss of the upper bridge arm of phase A in accordance with the actual parameters.The parameters of experiment platform are as follows.

TABLE I PARAMETERS OF WINDING ASYNCHRONOUS

MOTOR Rated power Pd (kW) 475

Stator voltage sU (V) 6000

Stator current dI (A) 59

Rotor voltage rU (V) 640

Rotor current rI (A) 435

Rated speed (r/min) 735 Power factor 0.85

TABLE II COMPONENTS PARAMETERS OF INVERTER MAIN

CIRCUIT

dcU 1100 V

DC-link capacitor parameters 1800μF /1300V

Power device parameters Infineon, FF1400R17IE4 series

Switching frequency 2000Hz The Well depth is 348m. Meanwhile, the lifting conditions

of one cycle is shown in Fig. 4.

a(m/s 2)

S(m)

t(s)

Current(A)

t(s)

V（m/s）

V1=0.2m/s

V2=1.5m/s

V3=5.8m/s

V4=1.0m/s

V5=O.3m/s

V6=O.3m/s

0

0.6

3

783

0.3

3.7

4.33

652

0.5

7

522

28.6

0

272

46.9

435

0.47

35

10.5

522

0.09

5

7.7

652

0

0.5

2

783 Fig. 4. Lifting conditions of one cycle of mine hoist

First of all, according to the FF1400R17IE4 datasheet and the power-loss calculation theory described in the former section, the relationship among power loss, load current and load impedance angle upper bridge arm in phase A can be obtained and was shown in Fig 5.

(a)

0 100 200 300 400 500 600 700 800

01

23

40

200

400

600

800

Ipk(A)Phi(rad)

PT

1(W

)

6Journal of Power Electronics, Vol. 12, No. 4, July 2012

(b)

(c)

(d)

(e)

Fig. 5. Relationship among power loss, load current and load impedance angle of upper bridge arm in phase A

All the dynamic power losses of the upper bridge arm in phase A can be illustrated in fig.6.

Fig. 6. Dynamic power-loss curves of the upper bridge arm in phase A

According to the traditional average power loss calculation of inverters in [11], the total average power loss of the NPC three-level inverter is 7380W, which will act as the total heat source during the process of forced-air cooling system design.

III. FORCED-AIR COOLING SYSTEM

DESIGN

The total heat source calculated in the chapter above will be the reference power during the theory analysis and design of forced-air cooling system. As noted in[23], the performance and weight of heatsink-fan forced-air cooling system mainly depends on the parameters shown in Fig. 7.

Total thermal resistance

Dimensions of heatsink

Weight of heatsink

Dimensions of duct

Air flow Cooling fan Weight of cooling fan

Total weight of air-forced cooling

system

Fig. 7.Factors affecting the thermal resistance and weight of heatsink-fan air-forced cooling system

The volume and weight of duct and housing are always

affected by many factors and have few difference under the same technological level, so they are out of consideration of this paper. What's more, the heatsink material in this paper is aluminum, and the material properties including thermal conductivity, thermal conductivity of air flow and density of heatsink material is not in the research object. Parameters in TABLE III are the factors used in cooling system design.

TABLE III DESIGN FACTORS OF COOLING SYSTEM

Detailed parameters Heatsinkgeometry Fin height, fin width, number of

fins, heatsink length, baseplate thickness, channel width

Duct geometry Duct width, duct length, duct height Fan Air flow curve, fan weight,

dimension It can be seen from TABLE IIIthat heatsink, duct and fan

affect the cooling capacity of the whole cooling system, and meanwhile, they are the most important factors influencing the total weight and power density of inverter. Under the

0200

400600

800

0

2

40

100

200

300

400

500

Ipk(A)phi(rad)

PD

1(W

)

0200

400600

800

01

23

40

200

400

600

Ipk(A)phi(rad)

PT2

(W)

0200

400600

800

01

23

40

100

200

300

Ipk(A)phi(rad)

PD

2(W

)

0200

400600

800

01

23

40

100

200

300

Ipk(A)phi(rad)

PD

5(W

)

0 20 40 60 80 100 120 140 160 1800

500

1000

1500

Time （s）

Pow

er L

oss

（ W）

T1

T2

D5

D1D2

TheOptimized Design of NPC Three-Level Inverter Forced-Air Cooling System…7

premise of constant thermal conductivity of aluminum and heat source of power devices, how to design fin thickness reasonably to reduce the weight of heatsink as much as possible and increase the number of fins as large as possible to improve the total fin surface plays an important role in the process of cooling system design and optimization. It must point out that the significant improvement of fin weight and fin width is limited by the manufacturing capacity and cost[24]. On the other hand, the cooling fan is an important factor influencing on both convection and total weight, which will affect the system structure and power density on the contrary. A set of air flow curves of standard fans available within this power range is shown in Fig.8. Although it is not like the aspects of fan volume, air flow, and static pressure, which have some theoretical formulas to support the design, on the basis of technical manuals and industry experience, the fan with a much larger air flow rate and much higher static pressure means a much heavier fan. Whether the fan has the minimum weight and meet the demand of cooling capacity or not decides the thermal stability of inverter. What's more, the longitudinal size, total weight and the most important power density of inverter also determined by the chosen fan.

2000 4000 6000 8000

0.5

1.0

1.5

2.0

2.5

0[CFM]

[In

H2O]

2000 4000 6000 8000 100001200014000V

200

400

600

800

1000

1200

1400

[m3/h]

[Pa]

∆ Pfa

1

2

3

4

5

6

7

8

9

10

11

12

140V 180V 230V 400V Fig.8. Relationship curves among fan power, air flow, and static pressure

According to [25], the total thermal resistance of the whole cooling system, including the thermal resistances of baseplate and fins, can be presented as

1

( 1)total

fin

bR nk L W h n s LR

= +⋅ ⋅ + ⋅ − ⋅ ⋅

(15)

where b is the baseplate thickness, k is the thermal

conductivity of aluminum, L is the length of heatsink,W is

the width of heatsink, h is the convective heat transfer

coefficient, n is the number of fins, and finR is the thermal

resistance of a single fin.

From [9], finR can be expressed as

1tanh( )fin

c c

Rh P k A m a

=⋅ ⋅ ⋅ ⋅

(16)

where m stands for the fin parameter, and can be expressed as follow

c

c

h Pmk A⋅

=⋅

. (17)

In (16) and (17), cP is the perimeter of fin, cA is the

cross-sectional area of fin, m is the fin parameter, and a is

the height of fins. cP and cA come from the equations as

follows respectively.

2 2cP a d= + (18)

cA a d= ⋅ . (19)

During the construction of the thermal model, the most significant parameter is h , and it can be expressed as in [17]

fNu kh

s⋅

= . (20)

In (20), fk is the fluid thermal conductivity and Nu is

the Nusselt number, which means the ratio of convective to conductive heat transfer across the convection boundary. As shown in [17] by Teertstra et al. Nu can be calculated by the following equation

13 33 1*

* 3r *

3.650.664 12s r

s

s

Re PNu Re PRe

−−− = + +

(21)

where, rP is the Prandtl number, standing for the ratio of

viscosity and thermal diffusivity. *sRe is the adjusted

channel Reynolds number, one channel parameter, and can be figured out from the following expression.

* ss

Re sReL⋅

= . (22)

In (22), sRe is the Reynolds number, indicating the ratio

of inertial forces to viscous forces, and can be presented as

chs

s VRev⋅

= . (23)

In this equation, s is the channel width, chV is the average

channel velocity, and v is the kinematic viscosity of the air.

8Journal of Power Electronics, Vol. 12, No. 4, July 2012

And meanwhile, the pressure drop of heatsink proposed ahead will be calculated from the equation below

2(2 )( )2

chapp c e

Vn a L s LP f K Ka W

ρ ⋅⋅ ⋅ + ⋅∆ = ⋅ + +

⋅(24)

where, appf is the apparent friction factor for a

hydrodynamically developing flow, cK is the coefficients

of a sudden contraction of heatsink channel, eK is the

coefficient of a sudden expansion of heatsink channel, and

ρ is the air density. appf for a rectangular channel can be

evaluated by using the following laminar flow formulation developed in [8] .

( )22

2

*

1 3.44 eeapp Dh

Dh

f f RR L

= + ⋅

. (25)

In (25), eDhR is the channel Reynolds number, *L is the

adjusted length of channel, and f is the friction factor.

*eh h

LLD R D

= (26)

2h

s aDs a⋅ ⋅

=+

(27)

e ch hh

V DR Dv⋅

= (28)

In aforementioned equations, hD is the hydraulic diameter

of the channel. In (24), the coefficients cK and eK can be

expressed as(from [20] and [25]) 2

0.42 1 1cn dKW

⋅ = − −

(29)

and 22

1 1en dKW

⋅ = − −

. (30)

eDhf R⋅ can be expressed as 2

3 4 5

e 24 32.527 46.721

40.829 22.954 6.089

Dhs sf Ra a

s s sa a a

⋅ = − +

− + −

(31) In accordance with [13], an approximation equation to

express the correlation of channel velocity and free stream velocity with bypass air flow in the duct can be obtained as

0.1251 1[1 ( ) ]ch f

s dV V L as+ = − ⋅

. (32)

In (32), fV is the approaching speed, 1L is duct

dimensionless length, and 1a is the ratio of bypass area and

single channel area. They are defined as

1 ed hd

LLR D

=⋅

(33)

2 DW DHDW DHhdD ⋅ ⋅

=+

(34)

1DW DHa

s a×

=⋅

(35)

where DW and DH are the width and height of duct respectively.

Generally speaking, the pressure drop in heatsink and duct geometry is an important factor that impacts the required air pressure of the fan, and an empirical expansion coefficient was adopted from [23] to indicate the relationship

222

, 1DH DW

fane fan

LK

= − ⋅

. (36)

In (36), ,e fanK is the coefficient of a sudden expansion

of the fan, and fanL is the length of the fan frame. It can be

seen from [23] that (36) describes the air flow expansion with a considerable accuracy (<10% error, analytical calculation compared with FEA simulation results).

2e,fan fan

fan 2K V

P Pρ⋅ ⋅

= ∆ + . (37)

Put (36) into (37), and the air flow of the fan can be expressed as

fan DH DWfV V= ⋅ ⋅ . (38)

Both fanP in (37) and fanV (38) constitute the operation

point of the fan, which must be on the air flow curve. The total thermal resistance can be obtained by solving these equations above, and the total weight of the heatsink-fan cooling system can be expressed by the following equation:

( ) fantotal hW b L W n a d L Wρ= ⋅ ⋅ + ⋅ ⋅ ⋅ + (39)

where, totalW and fanW stand for the total weight of the

cooling system and the fan weight respectively. TABLEIV

DESIGN FACTORS OF COOLING SYSTEM Design variable , , , , , chn d W L a v

TheOptimized Design of NPC Three-Level Inverter Forced-Air Cooling System…9

Design target Minimum totalW

Design limits n is integer, 1n >

1mmDW d> >

mDW W W> >

mDL L L> >

1mmDW s> >

1mmDH s> >

0DH a> >

0chV >

( ) /total c aR T T P< −

fanP and fanV should be

on fan curve TABLEV

CASE STUDY PARAMETERS Item Description

Power module Heatsink material Ambient temperature

Total average power loss: 7380W Junction temperature:125℃ Aluminum (200W/m-k) 30℃

Put the total power loss into these equations above as heat source, and the ideal heatsink structure and distribution of IGBTs can be obtained as shown in Fig.9.

207

0.5

±

612 0.25±

D5

IGBT1+FWD1

IGBT2+FWD2

IGBT3+FWD3

Fig. 9. The ideal heatsink structure and distribution of IGBTs based on the total average power loss

In Fig.9 IGBT3+FWD3 is the power device of the lower bridge arm of phase A. Put the average power loss of each power device in accordance with the dynamic power loss curve shown in Fig.6 into this distribution, and it can be figured out that the upper IGBT total power loss is 905W，the lower IGBT total power loss is 970W。

TABLEVI PARAMETERS OF HEATSINK

Material Aluminum (210W/m-k)

N (number of fins) 28

W (width of heatsink mm) 207 0.5± L (length of heatsink mm) 612 0.25± s (spacing between two fins mm) 6.5 a (height of fin mm) 101 0.15±

Put the total average power loss into cooling system design, and the pressure drop between inlet and outlet of heatsink can be figured out as follow

P=1075.78∆ Pa The approaching speed should meet the requirement as

follow

fV 8.5m/s>

Based on these two parameters, search the fan according to the operation point shown in Fig.8, and the fan with working characteristics at point ③can be used in this design, and its detailed parameters are shown in TABLEVII.

TABLEVII PARAMETERS OF FAN

Phase 3 Nominal voltage [V] 400 Connection Y Frequency[Hz] 50 Speed[min-1] 1375 Power input[W] 1430 Back pressure[Pa] 0 Air flow[m3/h] 8320 Sound pressure level[dB(A)] 78

IV. SIMULATION AND EXPERIMENT

The dynamic junction temperature curves of all power

devices in the upper bridge arm of phase A for two cycles was generated from the online thermal simulation tool of Infineon website, and was shown in Fig.10.

Fig.10. Junction temperature dynamic curves of phase A based on average power losses

Using finite element analysis software ANSYS to carry out the thermal analysis of cooling system, the simulation results are as follows.

0 20 40 60 80 100 120 140 160 18050

60

70

80

90

Time (s)

Tem

pera

ture

(

℃ )

Temperature of D5Temperature of T1

Temperature of T2

Temperature of D2

Temperature of D1

10Journal of Power Electronics, Vol. 12, No. 4, July 2012

(a)

(b)

(c)

Fig.11. Steady-state thermal simulation of the upper bridge arm of phase A in NPC three-level inverter.(a) Temperatures of power devices;(b) Inlet wind speed of heatsink;(c) Inlet pressure of heatsink

It can be seen from Fig.11 that the most high junction

temperature (76.1℃) of power devices is under the maximum temperature (125℃) of FF1400R17IE4 under normal working, the inlet wind speed of heatsink is much larger than 8.5 m/s, and the pressure drop between inlet and outlet of heatsink( the outlet connects to the negative pressure cavity, which can be considered as the 0 Pa reference point ) is 1174Pa close to the theoretical analysis result

P=1075.78∆ Pa. All of the simulations proves that the proposed heatsink

and fan model built based on the average power loss acting as heat source is suitable for this inverter to work in one cycle.

Winding asynchronous motor

NPC three-level inverter

Fig.12.The 1 MW experiment platform The experiment platform is shown in Fig.12. In order to

simulate the real working condition of the hoist, the temperature rise experiment of multicycle lifting will be made on this platform in accordance with the lifting conditions of one cycle in Fig. 4, and the duration is 1 hour. The experiment results are as follows.

1IG

BTU

(500

V/d

iv)

1IGBTU

(500

A/d

iv)

Out

put

I

(20ms/div)Time

Fig.13. Waveforms of steady-state operation of inverter

Line

U(5

00V

/div

)

(20ms/div)Time

Fig.14. Line Voltage Waveform of steady-state operation

TheOptimized Design of NPC Three-Level Inverter Forced-Air Cooling System…11

Roto

rU

(300

V/d

iv)

Torq

ueI

(150

A/d

iv)

Spee

dn

(300

r/div

)Re

activ

eI

(50A

/div

)

(10s/div)Time

Fig.15. Waveforms of Mine Hoist operating in one cycle

Fig.16. Wind speed of heatsink inlet

(a) Substrate temperature

(b) Surface temperature of power devices Fig.17. Temperatures of the upper bridge arm of phase A at

constant speed period

(a) Temperature of bridge arm back of phase A

(b) Surface temperature of T2 Fig.18. Temperatures of bridge arm back and T2 of phase A at

startup and deceleration stages It can be seen from Fig.13, Fig.14, and Fig.15 that the

system operates safely in one cycle with large currents in startup and deceleration stages. The wind speed of heatsink inlet during the process of system operation under normal

12Journal of Power Electronics, Vol. 12, No. 4, July 2012

conditions was shown in Fig.16, and the actual speed is 9.86m/s, larger than the theoretical value 8.5m/s. The initial temperature of inverter is the ambient temperature and after several lifting cycles the temperature will keep a dynamic balance at certain temperature level, which includes a rapid temperature climb at startup and deceleration stages, and a gradual decrease at the constant speed period. It can be seen from Fig.17 that the temperature of heatsink substrate fluctuate around 72℃ isothermal up and down within 2℃, and meanwhile the maximum surface temperature of power devices is 72℃. Temperature of bridge arm back of phase A shown in Fig.18 indicates that the area with the highest temperature is the location of lower IGBT of phase A, and its maximum surface temperature has reached 105 ℃ approximately. It can be deduced according to the engineering experience that the junction temperature is close to the maximum allowable temperature 125℃, which was confirmed by the heat accumulation damage of T2 at the 45th minute. The anatomy of T2 is shown in Fig.19.

Fig.19. Damage anatomy of power device caused by heat

accumulation From the analyses above, the conclusion can be drawn that

as a heavy load with periodic operation, using the average power loss as the heat source to design the cooling system is not suitable for the system to operate in continuous multicycle state with frequent overload. The reason is that using the average power loss as heat source means spreading the total power loss of power devices in one cycle on average, which will cover these special ranges generating large power losses in a short time, and during the multicycle operation the heat will accumulate gradually to damage the power devices eventually. In this mine hoist system, the special ranges are the deceleration period of former cycle and the acceleration

period of the next period, because the heat generated in the deceleration period will accumulate to the next cycle's accumulation, which will exceed the thermal capacity of cooling system and be a potential threat to power devices. Both the two ranges constitute the maximum power-loss range and according to the dynamic power-loss curves extract the average power losses of all power devices as the correction power to optimize the cooling system. From the simulation and experiment results above, it can be found that the total power of fan is enough for this inverter and all we should do right now is to optimize the size and shape of heatsinks within the requirements of theoretical analysis to keep a higher power density. The optimization results are as follows

TABLEVIII PARAMETERS OF OPTIMIZED HEATSINK

Material Aluminum (210W/m-k)

N (Number of fins) 48 W (width of heatsink mm) 360 0.5± L (length of heatsink mm) 612 0.25± s（spacing between two fins mm） 6.5 a（height of fin mm） 101 0.15±

D5IGBT1+FWD1IGBT2+FWD2

360

0.5

±

612 0.25±

IGBT3+FWD3IGBT4+FWD4

D6

Fig.20.Theheatsink structure and distribution of power devices after optimization

The heatsink model shown in Fig.20 is different from the model in Fig. 1 and Fig. 9 in some aspects including that the heatsink number of one bridge arm changes from 3to 2, all the power devices are distributed in one heatsink, the width of each heatsink increases153mm and the number of fins in one heatsink increase 12. It can be figured out that the pressure of heatsink inlet increase 14.3% compared with the old one, and the experiment results are as follows.

TheOptimized Design of NPC Three-Level Inverter Forced-Air Cooling System…13

Fig.21. The wind speed of optimized heatsink

(a) Substrate temperature

(b) Surface temperature of power devices

(c) Temperature of bridge arm back of phase A

(d) Surface temperature of T2 Fig.22. The experiment results after optimization It can be seen from Fig.21 and Fig.22 that the inlet wind

speed of the optimized heatsink is 11.20 m/s, and has a 13.5% increase rate compared with the former heatsink; the maximum temperature of the optimized heatsink is 62℃, and has a 16.2% decrease rate compared with the former one; the maximum temperature of T1 module surface 71℃, and has a 6.6% decrease rate compared with the former one; the maximum instantaneous temperature of T2 module surface at startup and deceleration stages is 68℃, and has a 35.2% decrease rate compared with the former one. From the results comparisons, the conclusion can be drawn that with the fin number of optimized heatsink increases, the wind resistance is decreased, the pressure drop between inlet and outlet of heatsink is enhanced, which improves the wind speed, and finally the heat capacity is upgraded. The experiment results shows that T2 has the largest temperature reduction and operate safely during the whole temperature rise test, which proves the optimization design of cooling System based on dynamic power loss calculation of maximum power-loss range is effective and feasible.

14Journal of Power Electronics, Vol. 12, No. 4, July 2012

V. CONCLUSION

In this paper, the average power loss of a 1MW mine hoist were calculated using the fast power-loss calculation method considered as the heat source to design the cooling system firstly. Based on theoretical analyses, the forced-air cooling system model was designed, and was proved by simulation. But as a multicycle load, the system has a weakness of instantaneous heat accumulation after continuous overload operation. Finally, according to the analysis of the periodic working condition, the maximum power-loss range was obtained and its dynamic power loss was taken into the optimized cooling system model as the heat source to solve instantaneous heat accumulation. The effectiveness and feasibility of optimized design for NPC three-level inverter cooling system based on the dynamic power loss calculation of the maximum power-loss range was proved by experiments.

ACKNOWLEDGMENT

The authors would like to thank 2014 Jiangsu Province Natural Science Foundation (BK20140204), the Research and Innovation Program of Postgraduates in Jiangsu Province (CXZZ13_0930) and the Fundamental Research Funds for the Central Universities(2012LWB73).

REFERENCES

[1] P. M.Fabis, D.Shum, and H.Windischmann,"Thermal modeling of diamond-based power electronics packaging," Annual IEEE Semiconductor Thermal Measurement & Management Symposium, pp.98-104, 1999.

[2] P. Mao，S.J. Xie, and Z. G. XU, “Switching transients model and loss analysis of IGBT module,” in Proc. CSEE, Vol. 30, No. 15 ,pp. 40-47, 2010.

[3] D. Yi，M. Y. Liao, “Losses Calculation of IGBT Module and Heat Dissipation System Design of Inverters,”Electric Drive Automation,Vol.24, No.3 , pp.159-163, 2011.

[4] A. D. Rajapakse ， A. M. Gole, and P. L. Wilson,“Electromagnetic transients simulation models for accurate representation of switching losses and thermal performance in power electronic systems,” IEEE Trans. Power Del., Vol. 20, No. 1 , pp.319-327, 2005.

[5] F. Krismer, J. W. Kolar, “Accurate power loss model derivation of a high-current dual active bridge converter for an automotive application,”IEEE Trans. Ind. Electron.,Vol. 57, No.3 , pp. 881-891, Mar. 2010.

[6] Q. Chen,“Analysis of Switching Losses in Diode-Clamped Three-Level Converter,”Trans. China Electrotechnical Society, Vol. 32, No.2, pp.68-75, Feb. 2008.

[7] M. H. Bierhoff and F. W. Fuchs, “Semiconductor losses in voltage source and current source IGBT converters based on analytical derivation,”in Proc.Power Electronics Specialists Conference, IEEE 35th Annual, Vol.4, pp. 2836-2842,2004.

[8] T. J. Kim，D. W. Kang，Y. H. Lee，and D. S. Hyun,“The analysis of conduction and switching losses in multi-level inverter system,”in Proc. Power Electronics Specialists Conference, IEEE 32nd Annual, Vol. 3, pp. 1363-1368,2001.

[9] Q. J. Wang, Q. Chen, W. D. Jiang, and C. G. Hu, “Analysis of Conduction Losses in Neutral-Point-Clamped Three-Level Inverter,” Trans. China Electrotechnical Society, Vol. 22, No.3, pp.66-70, Mar. 2007.

[10] S. Dieckerhoff, S. Bernet，D.Krug, “Power loss-oriented evaluation of high voltage IGBTs and multilevel converters in transformerless traction applications,” IEEE Trans. Power Electron., Vol. 20, No. 6, pp. 1328-1336, Nov. 2005.

[11] W.Jing, G. J. Tan, and Z. B. Ye, “Losses calculation and heat dissipation analysis of high-power three-level converters,” Trans. China Electrotechnical Society, Vol. 26, No. 2, pp. 134-140, Feb. 2011.

[12] K.Ma,Y. Yang，F. Blaabjerg,“Transient modelling of loss and thermal dynamics in power semiconductor devices,”in Proc. ECCE, IEEE, pp. 5495-5501,2014.

[13] W.Jing, “Study on Power Device Losses of High-Power Three-Level Converter,”Ph.D. Dissertation, China University of Mining and Technology, china, 2011.

[14] J. h. Hu, J. G. Li, and J. B. Zou, “Losses calculation of IGBT module and heat dissipation system design of inverters,”Trans. China Electrotechnical Society,Vol.24, No.3, pp. 159-163, Mar. 2009.

[15] F. Hong, R. Z. Shan，H. Z. Wang，Y. G. Yan, “Analysis and calculation of inverter power loss,”in Proc. CSEE, Vol.28, No.15, pp. 72-78, 2008.

[16] X. He，Y. Wu，H. Luo，P. Li，W. Li et. al., “Quasi-online modeling method of the power inverter losses based on igbt offline test platform,”Trans. of China Electrotechnical Society, Vol.29, No.6, pp. 1-6, Jun. 2014.

[17] U. Drofenik，G. Laimer，J. W. Kolar, “Theoretical converter power density limits for forced convection cooling,”in Proc.PCIM, pp. 608-619, 2005.

[18] U Drofenik，JW Kolar, “Analyzing the theoretical limits of forced air-cooling by employing advanced composite materials with thermal conductivities>400W/mK,”in Proc. CIPS, pp.1-6,2006 .

[19] E. M. Sparrow，B. R. Baliga，S. V. Patankar, “Forced convection heat transfer from a shrouded fin array with and without tip clearance,”Journal of Heat Transfer,Vol. 100, No.4, pp. 572-579,Aug. 1978.

[20] S.Lee, “Optimum design and selection of heat sinks,”IEEE Trans.Compon. Packag. Technol.,Vol. 18, No.4, pp. 812-817, Dec. 1995.

[21] W. Leonard，P. Teertstra，J. R. Culham，and A. Zaghol, “Characterization of Heat Sink Flow Bypass in Plate Fin Heat Sinks,”in Proc.ASME 2002 International Mechanical Engineering Congress and Exposition, pp. 189-196,2002.

[22] R. Hossain， J. R. Culham， and M. M. Yovanovich, “Influence of bypass on flow through plate fin heat sinks,”in Proc.Semiconductor Thermal Measurement and Management Symposium, Twenty Third Annual IEEE, pp. 220-227, 2007.

[23] P. Q. Ning，F. Wang，and K. D. T. Ngo, “Forced-air cooling system design under weight constraint for high-temperature SiC converter,”IEEE Trans. Power Electron., Vol. 4, No.29, pp. 1998-2007, Apr. 2014.

TheOptimized Design of NPC Three-Level Inverter Forced-Air Cooling System…15

[24] R. A. Wirtz，W. Chen，and R. Zhou, “Effect of flow bypass on the performance of longitudinal fin heat sinks,”Journal of Electronic Packaging,Vol. 116, No.3, pp. 206-211, Mar. 1994.

[25] J. R. Culham and Y. S. Muzychka, “Optimization of plate fin heat sinks using entropy generation minimization,” IEEE Trans.Compon. Packag. Technol., Vol. 24, No. 2, pp. 159-165, Jun. 2001.

Shi-Zhou Xu was born in Henan

province, China, in 1985. He received the

B.S. and M.S. degree of electrical

engineering from Henan University of Urban

construction (Pingdingshan, China) and

China University of Mining and Technology

(Xuzhou, China) respectively in 2009 and 2012.

He is currently working toward the Ph.D. degree in

electrical engineering at the Department of Information

and Electrical Engineering, China University of Mining

and Technology. His current research interests include

cooling system optimization, laminated busbar research,

and high-power three-level inverter modeling, control

and improvement.

Feng-You He was born in Zhangjiakou,

Hebei, China, in 1963. He received the B.S.

degree of automation from China University

of Mining and Technology, China, in 1984,

and the M.S. and Ph.D. degrees of power

electronics and power drives from China

University of Mining and Technology, China,

in 1992 and 1995, respectively.

Since 1984, he has been with the Department of Information

and Electrical Engineering, China University of Mining and

Technology, China, where he is currently a Professor. His

current research interests include the improvement of inverters,

advanced control of electrical machines, and power electronics.