Reaktiv effekt styrs av generatorns magnetiseringssystem.

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Teknisk- naturvetenskaplig fakultet UTH-enheten

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Populärvetenskaplig sammanfattning

Elnätet är ett komplext system som kräver en balans mellan förbrukad och konsumerad effekt. Det levereras hela tiden aktiv och reaktiv effekt till konsumenter, vars effekter kan förändras momentant och påverka frekvens och spänningsnivå i elnätet. För att bibehålla en jämn frekvens och spänningsnivå krävs snabb regleringsteknik. En förändring i aktiv effekt påverkar frekvensen i elnätet medan en förändring i reaktiv effekt påverkar spänningsnivån.

Aktiv effekt regleras av att öka/minska flödet genom turbinen som driver generatorn.

Reaktiv effekt styrs av generatorns magnetiseringssystem.

Syftet med ett magnetiseringssystem är att styra generatorns spänningsnivå. Genom att driva en ström genom rotorlindningen i generatorn kommer det att induceras en viss spänning i statorlindningen, det är den spänningsnivå generatorn levererar till nätet via en

transformator. Generatorns och elnätets spänningsnivå ska hela tiden stämma överens, därför anpassar magnetiseringssystemet rotorströmmen efter mätvärden från elnätet. Detta sker genom en regulatorfunktion kallad AVR (Automatic Voltage Regulator). Regulatorn jämför nätets spänning med generatorspänningen och utefter det skickas en styrsignal som anger om rotorströmmen ska öka eller minska.

Styrsignalen skickas till en drivarkrets som slår av och på elektriska switchar som är baserade på halvledarmaterial. Sådana komponenter är förknippade med förluster, vilka avges i form av värme. Om inte värmen leds bort riskerar dessa komponenter att bli dysfunktionella. Därför bör värmeförlusterna uppskattas för att säkerställa att magnetiseringssystemet klarar av att driva den tänka strömmen.

Det finns olika sorters magnetiseringssystem, statiska och borstlösa. Det statiska systemet använder sig ofta av en likriktare baserad på tyristorer för att styra rotorströmmen. För att överföra strömmen till generatorns rotor används mekanisk kontakt. I det borstlösa systemet används ingen mekanisk kontakt, utan strömmen förs över till rotorn genom induktion. För att styra strömmen används ofta en likriktare baserad på dioder och en DC-DC konverterare baserad på IGBT:er (Insulated Gate Bipolar Transistor), dessa komponenter benämns tillsammans för fältmatare.

I detta examensarbete har värmeutveckling undersökts för två fältmatare tillverkade av

Voith Hydro AB. Den ena är märkt 30 A vid kontinuerlig drift och den andra 150 A vid

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kontinuerlig drift. Syftet var att skapa en modell som kan simulera temperaturen i olika delar av fältmataren med naturlig kylning, alternativt forcerad kylning (med fläkt). Genom

simulering skulle de två fältmatarnas driftbegränsningar avgöras.

En termisk modell utvecklades i Microsoft Visual Basic for Applications och Excel för att beräkna temperaturerna i halvledarkomponenterna då de leder en viss ström. Modellen beräknar förluster som sker när komponenterna leder en ström, samt förluster som äger rum då komponenten slår av och på. Genom att använda sig av värmeledningstal angett i

datablad för komponenterna kunde en temperatur beräknas. Detta gjordes för en kontinuerlig ström och för en överström på 10 sekunder.

Den termiska modellen validerades genom experiment. En labbutrustning togs fram motsvarande ett magnetiseringssystem där alla komponenter monterades inuti ett elskåp.

Temperaturmätningar utfördes på olika delar av halvledarkomponenterna, kylfläns och omgivningen. Dessa mätningar jämfördes med de simulerade värdena. Värmeledningstal och förluster justerades utefter mätvärden för att ge en trovärdig modell.

Slutligen simulerades den maximala strömmen de två fältmatarna kan leda utan att överskrida några temperaturgränser. Resultatet av examensarbetet är att fältmataren PWM-30A, kan leda en ström på 30 A kontinuerligt med en överström på 60 A i

10 sekunder vid en omgivningstemperatur på 50 °C. I fallet då denna fältmatare kyls med

fläkt, klarar den av att leda en kontinuerlig ström på 50 A och 100 A i 10 sekunder vid en

omgivningstemperatur på 50 °C. Utan vidare studier ansågs fältmataren PWM-150A inte

kunna leda en högre ström än den ursprungligen var konstruerad för.

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Title Document ID Revision

Thermal modeling of power electronic components in excitation systems

R 7000 013 1

Prepared by Approved by Hydro structure Page

Fredrik Widberg Erik Dahlén AES0702 i

Executive summary

In this thesis work thermal modelling has been done for the field exciters PWM-30A and PWM-150A. The purpose was to develop a model that could be used to predict temperatures within the field exciters, and via simulation determine the maximum current that can be conducted through each product without exceeding any temperature limits.

Modelling of the PWM-30A was done with the option of cooling it naturally or by forced convection. PWM-150 was only simulated with forced convection since this product is intended to only operate with a fan.

Experiments were also carried out to validate the thermal model, this was done by

measuring temperature and conduction losses as a function of field current of different parts of the field exciters.

This thesis concludes that the PWM-30A can operate with a continuous current of 30 A, and

60 A for 10 seconds at an ambient temperature of 50 °C. When the PWM-30A is cooled by

forced convection, it can conduct a continuous current of 50 A at an ambient temperature of

50 °C. During field forcing the PWM-30A can conduct a current of 100 A for 10 seconds. It

has been concluded that without further testing the PWM-150A cannot conduct a larger

current than it was originally designed for, which is 150 A continuously at an ambient

temperature of 40 °C. During field forcing PWM-150A can conduct a current of 240 A for

10 seconds.

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R 7000 013 1

Fredrik Widberg Erik Dahlén AES0702 ii

1.Introduction ... 1

1.1 Background ... 1

1.2 Project description ... 4

1.3 Aim of the project ... 4

1.4 Project boundaries ... 4

1.5 Approach ... 5

2.Product description ... 6

2.1 PWM-30A ... 6

2.2 PWM-150A ... 7

2.3 Circuit diagram ... 9

3. Literature survey ... 10

3.1 Three-phase diode rectifier ... 10

3.2 DC-DC converter ... 12

3.3 The Diode ... 13

3.3.1 Forward characteristics of the diode ... 15

3.3.2 Power losses for a diode ... 16

3.4 The IGBT ... 18

3.4.1 Power losses for an IGBT module ... 18

3.5 Thermal management ... 21

3.5.1 Basic heat transfer theory ... 21

3.5.2 Thermal equivalent circuit ... 23

4. The thermal model ... 28

4.1 Software and user interface ... 28

4.2 Assumptions and limitation ... 29

4.3 Model structure ... 30

4.3.1 Power loss calculations ... 30

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Fredrik Widberg Erik Dahlén AES0702 iii

4.3.2 Temperature calculations ... 32

5. Desription of the experimental equipment ... 33

5.1 PWM-30A control system ... 34

5.2 PWM-150A cabinet ... 37

5.3 Source and load used for the experiment ... 39

5.4 Software used for measurements ... 40

6. Execution of the experiment ... 41

6.1 PWM-30A ... 41

6.2 PWM-150A ... 42

7. Adjustment of the model parameters ... 43

7.1 Thermal resistance PWM-30A with forced convection ... 44

7.2 Thermal resistance PWM-30A with natural convection ... 45

7.3 Thermal resistance PWM-150A ... 46

8. Validation of the thermal model ... 47

8.1 Power loss validation PWM-30A with forced convection ... 47

8.2 Temperature validation PWM-30A with forced convection ... 49

8.3 Power loss validation PWM-30A with natural convection ... 51

8.4 Temperature validation PWM-30A with natural convection ... 52

8.5 Power loss validation PWM-150A ... 54

8.6 Temperature validation PWM-150A ... 55

9. Simulation of the thermal model ... 57

9.1 PWM-30A operating limits with forced convection ... 58

9.2 PWM-30A operating limits with natural convection ... 59

9.3 PWM-150A operating limits ... 60

10. Discussion ... 61

11. Conclusion ... 68

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Fredrik Widberg Erik Dahlén AES0702 iv

12. Future work ... 69

13. References ... 70

14. Appendix ... 72

14.1 PWM-30A with forced convection ... 72

14.2 PWM-30A with natural convection ... 77

14.3 PWM-150A ... 81

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R 7000 013 1

Fredrik Widberg Erik Dahlén AES0702 v

Notations

Abbreviations

AC Alternating current DC Direct current

IGBT Insulated Gate Bipolar Transistor PLC Programable logic controller AVR Automatic voltage regulator PWM Pulse width modulation Nomenclatures

A Surface area that is conducting heat D Duty cycle for the IGBT

E

on

Energy dissipation when turning on the IGBT E

off

Energy dissipation when turning off the IGBT f

s

Switching frequency

i

d

Instantaneous diode current I

d

Average diode current

I

IGBT

Average IGBT current

I

load

Average load current

I

rect

Average output current from the rectifier bridge I

RRM

Maximum reverse recovery current in the diode

I

sw,avg

Average current through the switch in a chopper circuit P

d,cond

Average power loss while the diode conducts

P

d,RR

Average reverse recovery current power loss in the diode P

fwd

,

cond

Average conduction power loss in the freewheeling diode

P

fwd

,

sw

Average power loss due to switching losses in the freewheeling diode P

IGBT

,

sw

Average power loss due to switching losses in the IGBT

P

IGBT,cond

Average power loss for the IGBT while conducting Q Overall heat transfer

Q

cond

Heat transfer through conduction

Q

rad

Heat transfer through radiation

Q

conv

Heat transfer through convection

R

F,d

Forward slope resistance over the diode

R

F,fwd

Forward slope resistance over the freewheeling diode

R Thermal resistance

R Thermal resistance for conduction

R Thermal resistance for radiation

R Thermal resistance for convection

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Fredrik Widberg Erik Dahlén AES0702 vi

R

CE

Forward slope resistance for the IGBT R

-c

Thermal resistance from junction to case R

-s

Thermal resistance from case to sink R

-a

Thermal resistance from sink to ambient

T

a

Ambient temperature

t

b

Falltime for the maximum recovery current T

c

Case temperature

T

s

Heatsink temperature T

surf

Surface temperature T

j

Junction temperature V

Br

Breakdown voltage

v

CE

Collector emitter voltage when the IGBT is conducting V

DC

Output voltage from the rectifier bridge

v

F,d

Instantaneous forward voltage drop over diode

v

F,fwd

Instantaneous forward voltage over freewheeling diode V

m,L-L

Maximum line-to-line voltage at rectifier input

V

out,avg

Average output voltage from the DC chopper

V

Th,d

Threshold voltage for a diode

V

Th,fwd

Threshold voltage for a freewheeling diode Heat transfer coefficient

Emissivity

Heat conductivity Layer thickness

Stefan-Boltzmann constant

Time constant due to thermal heat capacity

Temperature difference over the conducting layer

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R 7000 013 1

Fredrik Widberg Erik Dahlén AES0702 1

1. Introduction 1.1 Background

The electric grid is a complex system. Active and reactive power are delivered to customers at a fixed frequency and voltage while the load changes continuously. This imposes the requirement for a quick regulation of the active and reactive power [1]. The synchronous generator is the most important component that helps to keep the electric grid stable. A synchronous generator consists of a fixed part, the stator, and a rotating part called rotor. The rotor is driven by some sort of

mechanical work. A change in active power affects the system frequency and must be compensated for. This is done by a speed governor that will adjust the input to the turbine. When there is a change in reactive power the electric grid voltage is affected. An excitation system compensates for these variations [1].

The purpose of an excitation system is to magnetize the generator and control the stator output voltage. This is done by injecting a controlled field current through the rotor winding which induces a certain voltage in the stator [2]. The power needed by the excitation system is often taken from the generator terminal bus, but can also be drawn from other sources. The voltage must be transformed to a manageable level and converted into DC. Rectification is usually made by a diode bridge rectifier or a thyristor bridge rectifier [2].

There are two main excitation systems used for synchronous generators, static and brushless

systems [2]. This thesis will only deal with the brushless system. Such a system transfers the current

to the rotor by induction [2]. Figure 1 shows an example of a brushless excitation system.

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R 7000 013 1

Fredrik Widberg Erik Dahlén AES0702 2

Figure 1: Example of a brushless excitation system made by Voith [2]

A brushless excitation system consists of many parts, as can be seen in Figure 1. The excitation system shown in the figure draws power from the generator terminal bus, or from the grid. The AC source is converted into DC by a diode rectifier. The rectifier cannot control the rotor current by itself. Therefore, a DC-DC converter is connected to the rectifier output. The DC-DC converter shown in Figure 1 is a step-down converter, this type of converter can only give an average output voltage that is less than the input. The average output voltage and current of the DC-DC converter is controlled by an IGBT that is turned on and off by the programmable logic controller (PLC). The diode rectifier and the DC-DC converter are together called a field exciter. The output current of the field exciter is DC, and is conducted through a winding that is fixed around the rotor shaft and is called the field exciter stator winding. By induction, the current is transferred into the rotor shaft.

The field exciter rotor winding is wound in such a way that the transferred DC current is turned into a three-phase AC waveform. In order for the generator to reach a certain output voltage the AC current needs to be rectified, therefore there is also a rectifier mounted on the rotor shaft. The output from the rectifier on the rotor shaft is then conducted through the rotor winding of the generator.

This enables a current and voltage to be induced in the stator of the generator. The current through

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Fredrik Widberg Erik Dahlén AES0702 3

an inductor cannot be changed instantaneously. Since the output current of the field exciter is transferred by induction in several steps, it will lead to a certain time delay before the current in the rotor winding changes. In order to maintain a quick regulation of the generator voltage, the output current of the field exciter is increased to a much higher value for a short duration. This minimizes the time delay, but it strains the equipment [2].

The field exciter used to control the rotor current is based on power electronic devices which are associated with losses and thus heat dissipation [3]. It is essential for the excitation system not to fail because this implies costs for the operator. Therefore, it is important to consider the power losses in the semiconductors and implement a sufficient cooling system to prevent device failure. It may happen if a semiconductor exceeds its temperature limit [2].

This thesis work will examine power loss and heat development in power electronics components

of field exciters made by Voith Hydro AB. A field exciter is shown in Figure 1 and it includes a

diode rectifier and a DC-DC converter.

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R 7000 013 1

Fredrik Widberg Erik Dahlén AES0702 4

1.2 Project description

The two field exciters studied in this thesis are commercial products of Voith Hydro AB which are called PWM-30A and PWM-150A. They consists of a diode rectifier in series with a DC-DC converter that only operates in the positive quadrant of the voltage-current plane. Both products are used in brushless excitation systems for control of the rotor current. The semiconductor components that are used for these products are associated with losses. They are mounted on a heatsink to protect them from overheating. The PWM-30A is, as standard, cooled by natural convection. For some applications, this might be insufficient. For that reason Voith wants to investigate the thermal limitations with both natural and forced convection for this product. The PWM-150A is built with a fan as standard and Voith wants to determine the thermal limitations for this product as well.

1.3 Aim of the project

The goal of the thesis is divided into two parts. The first aim is to develop a model that can be used to simulate the temperatures at different operating conditions for the two field exciters. The second aim is to use the model to simulate and extract the maximum current that can be conducted through the two field exciters. For the PWM-30A it is done for both natural and forced convection, for the PWM-150A only forced convection is implemented.

1.4 Project boundaries

The thermal model developed in this thesis was designed for a chopper based field exciter including a diode bridge and an IGBT module. Field exciters deviating from this structure were not implemented.

In the experimental part of the thesis work, the field exciters were not connected to a rotor winding. An inductive load was used as a substitute, but it was deemed to be close enough to an actual rotor winding.

This thesis work does not include any detailed description of the PLC- and panel software and how these were implemented for control of the experimental exciting equipment.

Dimensioning of cables and components included in the experimental exciting system are

not discussed in this thesis.

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Fredrik Widberg Erik Dahlén AES0702 5

1.5 Approach

Literature survey

The literature survey contains a section describing the function of three-phase rectifier and a step down converter. It also includes a description of the characteristics of basic

semiconductor devices, such as the diode and the IGBT. There is also a section describing the losses and thermal management for semiconductor devices.

Thermal model

This section includes a description of the model and how it was developed using the theory presented in the literature survey.

Description of the experimental equipment

The experimental equipment used to validate the thermal model is described in this section.

It includes a description of the control system for both the PWM-30A and the PWM-150A.

Execution of the experiment

This section describes the execution of the experiment and how the parameters of the thermal model were adjusted to fit the measured values.

Thermal model validation

This section describes how the validation of the model has been carried out.

Simulation of the thermal model

When the model had been validated, it was used to simulate and extract the operating limits

for the two field exciters. How this was done is described in this section.

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Fredrik Widberg Erik Dahlén AES0702 6

2. Product description 2.1 PWM-30A

The PWM-30A is a commercial product manufactured by Voith Hydro AB. Its purpose is to control the current conducted through the rotor winding of a generator, and thus the output voltage and the reactive power delivered to the electric grid. The power circuit consists of a three-phase rectifier and an IGBT module that is used to control the output current by switching it on and off at a frequency of 110 Hz. The PWM-30A is constructed to conduct a continuous current of 30 A and a current of 60 A for 10 seconds, given an ambient temperature of 50 °C. The 10 seconds of

overcurrent is called field forcing and it is necessary to meet certain requirements regarding the response time of the excitation system in case of disturbances. By rapidly increasing the current to a high level results in a fast response time when the exciting system compensates for changes in the electric grid. The PWM-30A is designed with the option of mounting a fan on top of the exciter where the fan sucks the air through the heatsink [2]. An overview of the field exciter PWM-30A is shown in Figure 2.

Figure 2: Overview of the field exciter PWM-30A

Figure 2 shows an overview of the PWM-30A, the components included are listed in Table 1.

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Fredrik Widberg Erik Dahlén AES0702 7

Table 1: PWM-30A, list of components

Component Article No. Manufacturer

1 PWM circuit board K7000 014 VOITH 2 IGBT module SKM150GAL12T4 Semikron

3 IGBT driver SKHI10/12R Semikron

4 Diode rectifier SKD 110/16 Semikron

5 Bleeding resistor Elfa

6 Capacitor 5 µF 850 V

DC

Arcotronics

7 Heatsink P3/180 Semikron

2.2 PWM-150A

The PWM-150A is a commercial product made by Voith Hydro AB. The power circuit of the

PWM-150A consists of an IGBT module that operates with a switching frequency of 110 Hz and

three diode modules which form the rectifier bridge. PWM-150A is designed to conduct a current

of 150 A continuously, and 240 A for 10 seconds at a surrounding air temperature of 40 °C. The

PWM-150A is contructed with a fan, the fan is mounted at the bottom of the exciter and blows the

air through the heatsink [2]. An overview of the field exciter PWM-150A is displayed in Figure 3.

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Fredrik Widberg Erik Dahlén AES0702 8

Figure 3: Overview of the field exciter PWM-150A

The PWM-150A is shown in Figure 3, the components included are listed Table 2.

Table 2: PWM-150A, list of components

Component Article No. Manufacturer

1 PWM circuit board K7000 014 VOITH 2 IGBT module SKM300GAL12T4 Semikron

3 IGBT driver SKHI10/12R Semikron

4 Diode rectifier SKKD 162/16 Semikron

5 Bleeding resistor Elfa

6 Capacitor 60 µF 1120 V

^DC

Westcode

7 Heatsink P3/330 Semikron

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Fredrik Widberg Erik Dahlén AES0702 9

2.3 Circuit diagram

Figure 4: PWM-30A and PWM-150A circuit diagram

The cicuit diagram for the PWM-30A and the PWM-150A is displayed in Figure 4. Diode D1 to D6 makes up the rectifier bridge with the incoming three-phase voltage denoted by U, V and W. The output of the rectifier is connected to a capacitor that is used to reduce ripple in the output voltage and current. The IGBT is an electronic switch that controls the output voltage and current.

Terminals F1 and F2 are normally connected to the exciter stator winding. The diode D7 is a

freewheeling diode that is conducting while the IGBT is turned off. It is necessary to prevent

voltage spikes due to stored magnetic energy in the rotor winding. The freewheeling diode is

integrated with the IGBT module. The bleeding resistor connected over the capacitor allows the

capacitor to discharge when the field exciter is turned off [2].

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Fredrik Widberg Erik Dahlén AES0702 10

3. Literature survey

This section reviews the basic features of a three-phase rectifier and a step-down converter. These converters are based on semiconductor devices such as the diode and the IGBT. Semiconductors are associated with electrical losses. These losses are dissipated as heat and therefore is there a section explaining heat transfer for semiconductors. This section also explains how the temperature in different parts of a semiconductor device can be calculated.

3.1 Three-phase diode rectifier

A three-phase rectifier is a common component for industrial applications. Its purpose is to convert an AC source to DC with as little voltage ripple as possible [3]. Figure 5 shows an ordinary three- phase rectifier with diode switches.

Figure 5: Three phase-rectifier circuit

In a three-phase rectifier, as displayed in Figure 5, only one diode in the top half can conduct at the

same time. The same holds for the lower half of the rectifier [3]. The diodes conduct in pairs where

the largest line-to-line voltage determines which diodes will conduct. This results in an output

voltage equal to the maximum line-to-line voltage of the source. It only applies if the rectifier is

connected to an infinite smoothing capacitor in parallel with the load [3]. Figure 6 shows the

conduction scheme for the diode pairs.

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Fredrik Widberg Erik Dahlén AES0702 11

Figure 6: Diode pair conduction scheme. The figure displays the line-to-neutral voltage of the source in the top, in the bottom the line-to-line voltage is displayed. The numbers also shows which diodes in the bridge that conduct at the same time [3]

Figure 6 illustrates the conduction scheme for a three-phase diode rectifier. This type of rectifier is called a six-pulse rectifier, because there is a transition of the highest line-to-line voltage every 60 and it will result in six pulses during a whole period. Therefore, the frequency of the output will be