Development of grid code handling process and simulation models for synchronous machines

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Tiia Moilanen

DEVELOPMENT OF GRID CODE HAN- DLING PROCESS AND SIMULATION

MODELS FOR SYNCHRONOUS MA- CHINES

Master’s thesis Faculty of Information Technology and Communication Sciences Petros Karamanakos

Sahas Shah

January 2022

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ABSTRACT

Tiia Moilanen: Development of Grid Code Handling Process and Simulation Models for Syn- chronous Machines

Master’s thesis Tampere University

Master’s Degree in Electrical Engineering January 2022

Synchronous machines are electrical machines that rotate with synchronous speed. Two types of synchronous machines are discussed: salient pole rotor synchronous machines and nonsalient or round rotor synchronous machines. A synchronous generator is designed based on IEC or NEMA standards. Sometimes the design based on these standards does not fulfil the requirements given by the grid codes and then additional modifications are needed. In grid code projects, salient pole rotor synchronous generators are mostly used, which is why they are more discussed in the thesis. An equivalent circuit, which is used to study performance characteristics of a synchronous machine, is explained in the thesis for both nonsalient and salient pole machines.

Grid codes are a set of regulations set by respective TSOs to all the connection points of the power grid. Grid codes have regulations for voltage and frequency variation, FRT (fault ride through), power quality etc. The grid codes vary between different TSOs and in European area ENTSO-E has set some common regulations for all the power grids operating in its area to ensure good power quality, safety, and reliability of the network. The grid code requirements for voltage and frequency variation and FRT of Finland, Italy and Germany are discussed in more detail in the thesis.

The goal of the thesis is to create simulation models for the support teams and customer and end-users. The simulation models are created using DIgSILENT PowerFactory program. The created simulation models are FRT simulation, load connection and rejection simulation and load flow simulation. The simulation models are used to run simulations in the pre-sales phase of new grid code projects. In the thesis, example simulations are run, and the simulation results are presented. In addition to the simulation models, an automation script for the generator object is created. The purpose of the automation script is to allow the user to give all generator object parameters for the simulation from an Excel worksheet without the need to fill the parameters manually one by one. The generator parameters are first added to the Excel worksheet. The script then reads the parameters in correct order from this worksheet.

Current simulation models for the simulations are created with MathWorks Simulink. They are used by the R&D team. With the created PowerFactory models, these simulations could be per- formed by the support team without the need to contact the R&D. Later the simulations could be run by the customer and the end-user themselves due to the simple interface of the PowerFactory program.

Another goal of the thesis is to develop the grid code handling process. The handling process is inspected and developed by creating two documents for the front-end and back-end teams.

The first document is a questionnaire, where the grid code project process related questions from the sales team members are collected and answered. The second document is an FAQ document, which includes questions coming often from the customer and the end-user. The questions are collected with the help of technical support team and a sales team member. The FAQ questions and answers are mostly related to machine design parameters and the documentation needed from the customer and end-user. These two documents help the sales team to provide offer requests faster and to answer questions coming from the customer and the end-user at the beginning of the grid code project. Another tool for faster response in the pre-sales phase is a statement of grid code compatibility. It can be provided when an offer request is received for a machine that has been previously proved to fulfil the same operation and grid code requirements.

Keywords: grid codes, synchronous machine, synchronous generator

The originality of this thesis has been checked using the Turnitin OriginalityCheck service.

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TIIVISTELMÄ

Tiia Moilanen: Development of Grid Code Handling Process and Simulation Models for Syn- chronous Machines

Diplomityö

Tampereen yliopisto

Sähkötekniikan diplomi-insinöörin tutkinto Tammikuu 2022

Tahtikoneet ovat sähkökoneita, jotka pyörivät tahtinopeudella. Tahtikoneita on sekä avonapaisia että umpinapaisia. Tahtigeneraattori suunnitellaan IEC- tai NEMA-standardien pohjalta. Jos- kus näihin standardeihin pohjautuva suunnittelu ei täytä verkkokoodien asettamia vaatimuksia.

Tällöin generaattorin rakennetta pitää muokata. Verkkokoodiprojekteissa käytetään enimmäk- seen avonapaisia tahtigeneraattoreita, minkä vuoksi tässä työssä keskitytään niihin. Sähköko- neita mallinnetaan sijaiskytkennän avulla ja tässä työssä käydään läpi sijaiskytkennät sekä avonapaiselle että umpinapaiselle tahtikoneelle.

Verkkokoodit ovat vaatimuksia, jotka siirtoverkonhaltijat ovat asettaneet kaikille sähköverkon liitäntäpisteille. Verkkokoodeissa on vaatimuksia jännitteen ja taajuuden vaihtelulle, FRT:lle (fault ride through), sähkön laadulle jne. Verkkokoodit vaihtelevat eri siirtoverkonhaltijoiden välillä, ja Euroopan alueen yhteinen toimija ENTSO-E on asettanut joitakin yhteisiä sääntöjä kaikille alu- eellaan toimiville sähköverkoille. Yhteiset säännöt varmistavat sähköverkon sähkönlaadun, tur- vallisuuden ja luotettavuuden. Tässä työssä käsitellään tarkemmin ENTSO-E:n jäsenmaista Suo- men, Italian ja Saksan verkkokoodien vaatimuksia.

Diplomityön tavoitteena on luoda simulointimallit teknisen tuen sekä loppukäyttäjien käyttöön.

Mallit tehdään DIgSILENT PowerFactory -ohjelmalla. Luodut simulointimallit ovat FRT-simulointi, kuorman kytkentä- ja hylkäyssimulointi sekä tehonjakosimulointi. Simulointimalleja käytetään si- mulointien suorittamiseen uusien verkkokoodiprojektien alkuvaiheessa. Työssä ajetaan esimerk- kisimulaatioita ja esitellään niiden tulokset. Simulointimallien lisäksi luodaan automaatiokoodi si- mulointimallin generaattoriobjektille. Automaation tarkoituksena on antaa käyttäjälle mahdollisuus syöttää kaikki generaattoriobjektin parametrit simulointia varten Excel-tiedostosta ilman, että niitä tarvitsee täyttää manuaalisesti. Generaattorin parametrien syöttämistä varten luodaan Excel-tau- lukko, mistä koodi lukee kaikki generaattorin parametrit.

Nykyiset käytössä olevat simulointimallit on tehty MathWorks Simulink -ohjelmalla. Niitä käyt- tää tällä hetkellä ainoastaan R&D-tiimi. PowerFactoryn luomilla malleilla simulointeja voisivat suo- rittaa ensin tekninen tuki ilman, että heidän tarvitsee ottaa yhteyttä R&D-tiimiin. Myöhemmin simulointimalleja voisi käyttää myös asiakas ja loppukäyttäjä PowerFactoryn simulaatioiden yksin- kertaisemman käyttöliittymän vuoksi.

Toinen työn tavoite on kehittää verkkokoodiprojektien käsittelyä ABB:llä. Tätä varten työssä luodaan kaksi dokumenttia myynti- ja tukitiimeille. Ensiksi myyntitiimin jäseniltä pyydetään verk- kokoodiprojekteihin liittyviä kysymyksiä. Nämä kysymykset ja niiden vastaukset kootaan sitten yhteiseen dokumenttiin. Seuraavaksi luodaan UKK-dokumentti (usein kysytyt kysymykset), joka sisältää usein asiakkailta ja loppukäyttäjiltä tulevia kysymyksiä. UKK-kysymykset kerätään teknisen tuen kanssa. UKK-kysymykset ja -vastaukset liittyvät useimmiten koneen parametreihin ja ominaisuuksiin sekä projektin teknisiin tietoihin, joita asiakkaalta ja loppukäyttäjältä tarvitaan tar- kempien simulaatioiden suorittamiseksi myöhemmässä vaiheessa projektia. Nämä kaksi dokumenttia auttavat myyntiä reagoimaan nopeammin asiakkaan ja loppukäyttäjän tarjouspyyntöihin ja kysymyksiin verkkokoodiprojektien alkuvaiheessa. Lisäksi projektin alkuvaiheessa tarjouksen antamista voidaan nopeuttaa, mikäli tarjottava kone on todettu jo aiemmassa projektissa vaatimukset täyttäväksi. Tarjouksen antamista nopeutetaan antamalla lausunto siitä, että kone on aiemmin todettu täyttävän kyseisen verkkokoodin vaatimukset.

Avainsanat: verkkokoodi, tahtikone, tahtigeneraattori

Tämän julkaisun alkuperäisyys on tarkastettu Turnitin OriginalityCheck –ohjelmalla.

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PREFACE

This thesis was conducted in ABB Pitäjänmäki factory under the R&D team. I want to thank everyone who helped with the thesis: Markku Vainamo, Iiro Uotila, Timo Nikkari, Andreas Doerr, Juho Laurila, Tuomas Janhunen, Juha Kinanen and everyone else in ABB, who took part in the thesis with either giving comments or assisting with difficult tasks or questionnaires. I especially want to thank my thesis supervisor Sahas Shah for guidance during the thesis and for the support given throughout the process. I also want to thank my Tampere University supervisor Petros Karamanakos for guidance with the thesis writing process.

The past years in the university have been filled with experiences, friends and learning.

I want to thank my family and friends for supporting me during and outside of my studies.

Tampere, 16 January 2022

Tiia Moilanen

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LIST OF FIGURES

Figure 1. Construction of synchronous machine stator and salient pole and

nonsalient pole rotors. [1] ... 3

Figure 2. Simple permanent magnet construction. ... 4

Figure 3. Cross-section of a generator with integrated shaft fan. [3]... 6

Figure 4. a) Cross sectional drawing and b) Dimension drawing of a generator. ... 7

Figure 5. Equivalent circuit and phasor diagram for (a) synchronous generator and (b) synchronous motor. [1] ... 9

Figure 6. Phasor diagrams of salient pole synchronous machine working in generator mode. [1] ... 10

Figure 7. Closeup of the transmission system network operated by members of ENTSO-E in Northern Europe. [4] ... 14

Figure 8. The requirements for frequency variation in different ENTSO-E operating areas. [7] ... 16

Figure 9. The FRT capability requirement graph for a synchronous power plant. [5]... 17

Figure 10. The FRT capability requirement graph for type D synchronous generator group for symmetrical faults. [8] ... 18

Figure 11. The FRT capability requirement graph for synchronous generator group. [10] ... 19

Figure 12. Comparison of IEC 60034-3 requirement of generator voltage- frequency limits with grid requirements of SvKFS. [11] ... 21

Figure 13. Simplified generator-grid connection diagram. [11] ... 22

Figure 14. PID-controller a) block diagram and b) implementation. [16] ... 23

Figure 15. A general view of PowerFactory simulation interface. ... 25

Figure 16. PowerFactory schematics of a) a synchronous generator and b) a general load. ... 26

Figure 17. A common model AVR used in the simulation. ... 26

Figure 18. Simulation model for FRT simulation... 27

Figure 19. Fault event selection for FRT simulation. ... 28

Figure 20. Simulation signals for FRT simulation without power flow to the grid. ... 28

Figure 21. Simulation signals for FRT simulation with power flow to the grid. ... 30

Figure 22. Simulation model for load connection and rejection simulation. ... 31

Figure 23. Simulation signals for load connection and rejection simulation. ... 32

Figure 24. Fault event selection for load connection simulation. ... 33

Figure 25. Simulation model for load flow simulation. ... 34

Figure 26. Simulation signals for load flow simulation. ... 35

Figure 27. Simulation signals for load flow simulation when the grid is disconnected. ... 36

Figure 28. Parameters and external objects given for the automation script. ... 37

Figure 29. Input window to give the Excel file address. ... 37

Figure 30. Excel worksheet for generator parameters for the script. ... 38

Figure 31. Simulink model for FRT simulation. ... 39

Figure 32. Simulink curve plotting window. ... 39

Figure 33. Generator voltage Ut and generator current It in Simulink simulation signals. ... 40

Figure 34. Process steps of grid code project handling in ABB. ... 41

Figure 35. Lead time change with created simulations models versus without the simulation models. ... 42

Figure 36. Kceil voltage calculation formula. [20] ... 45

Figure 37. Draft of the statement of generator grid code compliancy. ... 46

Figure 38. The map of transmission system network that is operated by members of ENTSO-E. [6] ... 52

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LIST OF SYMBOLS AND ABBREVIATIONS

AVR Automatic Voltage Regulator

ENTSO-E European Network of Transmission System Operators for Electricity

FRT Fault Ride Through

GTS Global Technical Support m.m.f. Magnetomotive force

PID Proportional Integral Derivative

p.u. Per Unit

R&D Research and Development ROCOF Rate of Change of Frequency

RPM Rounds per Minute

SCR Short-Circuit Ratio

TSO Transmission System Operator

𝐸_𝑓 excitation voltage

𝐸_𝑓^′ excitation voltage when saliency is neglected

𝑓 frequency

𝐹_𝑎 armature m.m.f.

𝐹_𝑓 field m.m.f.

𝐼_𝑎 armature current

𝐼_𝑏 base current

𝐼_𝑓 field current

𝐼_𝑑 D-axis current

𝐼_𝑞 Q-axis current

𝐾_𝐷 derivative gain

𝐾_𝐼 integral gain

𝐾_𝑃 proportional gain

Φ_𝑎 armature flux

Φ_𝑓 field flux

ϕ terminal power factor angle

𝜓 internal power factor angle

𝑝 number of poles

𝑅_𝑎 armature resistance

𝑅𝑃𝑀 rotational speed

𝑇_𝐷 lag time constant

𝑉_𝑡 terminal voltage

𝑋_𝑎𝑙 leakage reactance

𝑋𝑎𝑟 magnetizing reactance

𝑋_𝑑 D-axis reactance

𝑋_𝑠 synchronous reactance

𝑋_𝑞 Q-axis reactance

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1. INTRODUCTION

This thesis is conducted in ABB Oy in Helsinki Pitäjänmäki factory. The first goal of the thesis is to create simulation models. These models would be used by ABB sales support and customer and end-user during pre-sales phase and in the early order phase of new projects. The second goal of the thesis is to develop the process of handling new generator projects, which need to consider the grid codes. In addition to creating simulation models and developing the process handling, synchronous machine modelling and the basics of grid codes are explained.

The background information of synchronous machines and grid codes is given in chapters 2 and 3. First in chapter 2, synchronous machines and especially salient pole synchronous machine’s modelling, and construction are discussed. Synchronous machine’s rotor and stator magnetic field rotate at the same speed called synchronous speed. Sa- lient pole synchronous machines have non-uniform air gaps. Due to non-uniformity, the rotor poles are divided into D- and Q-axis. Modelling using D- and Q-axis is called DQ- modelling and it is discussed in the thesis. Understanding the principal operation of synchronous machines, and the necessary parameters needed in the modelling, is important when running the simulations.

In chapter 3 the principle of grid codes and some grid code requirements are discussed.

In this chapter the grid codes are first explained in more general level, and after that the grid codes in different areas are discussed in more detail. Grid codes are a set of regulations given by local transmission system operators (TSOs) for connections to the local grid. A closer look is taken into the grid codes set by ENTSO-E, which governs the grid codes in Europe. Grid codes set by ENTSO-E members in Finland, Italy and Germany are discussed in more detail. The TSOs in these countries are selected for the thesis, because they are common customer countries for ABB projects. Generators are manu- factured based on either IEC or NEMA standards. Sometimes these standards are not enough to fulfil customer needs, and then modifications to the generators are needed.

After the theory, in chapter 4, the simulations are created and further explained. Simula- tion models are created for fault ride through (FRT) simulation in chapter 4.1, load connection and rejection simulation in chapter 4.2 and load flow simulation in chapter 4.3.

The simulation models and the example results are presented for each simulation exam-

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ple. In addition to the simulations, a script is created for the generator object in the simulation model. With the script, a user could run the automation for an Excel worksheet, which contains all the generator parameters. Then the script transfers the parameters automatically for the generator object in the simulation model. Without the script, the user would have to add the generator parameters manually. The simulations and the script are done using DIgSILENT PowerFactory 2021 program. The program has a built-in component library. The components for the simulation models are selected from there.

The parameters for each component can be set by the user. The program allows the user to create events for the simulations and plot figures of the variable values, which the user wants to examine. Different events are created for different simulations. The simulation models are created firstly for the support team and possibly later for the ABB sales department and the customer and end-user. The models are used in the pre-sales phase of grid code projects.

The current simulation process with MathWorks Simulink is introduced for comparison to the PowerFactory program simulations. The Simulink simulation model for FRT is used currently by the R&D team. The rest of the simulations are also run with Simulink by R&D.

The last topic of the thesis is to develop the process of grid code project’s handling and it is discussed in chapter 5. To ease the process handling, a questionnaire is created based on questions received from the sales team members. These questions are then answered in the common questionnaire document. In addition to the questionnaire, also an FAQ document is created of the common questions the customer has during new grid code projects. These two documents help the sales team members to react faster to new offers of grid code projects in the pre-sales phase. Another document that could speed the offering process is a statement of grid code compatibility. The statement could be provided in case an offer request is made for a machine that has been previously proved to fulfil the same operation and grid code conditions.

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2. MODELLING OF SALIENT POLE SYNCHRO- NOUS MACHINE

Synchronous machines are electrical machines that rotate in constant synchronous speed in steady state operation. In synchronous speed both air gap field and rotor rotate at the same speed. Synchronous machines can be used as both motors and generators.

Motors convert electrical power into mechanical shaft power and generators convert the rotational mechanical power into electrical power. Synchronous machines are commonly used as generators, and they are the primary energy conversion devices in electric power systems in the world today. [1]

In Figure 1 is a construction of synchronous machine stator and two different kinds of rotors: salient pole rotor and cylindrical or nonsalient pole rotor. In this thesis, a closer look is taken into the salient pole construction as it is more commonly used in synchronous generators than nonsalient pole rotors. The salient pole construction is also more common in grid code projects.

Figure 1. Construction of synchronous machine stator and salient pole and nonsali- ent pole rotors. [1]

The rotation of electrical machine is based, in basic, on magnets. Magnets have always two poles: one pole is called south and one is called north. That is why electrical machines have always minimum two poles, or one pole pair. These poles are also marked in Figure 1 rotor constructions. The output frequency of a synchronous generator is proportional to the number of pole pairs. If one pole pair rotates 50 times in one second, the output frequency of the machine would be 50 Hz. To obtain the needed output frequency

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with given pole pairs, the rotational speed in RPM (rounds per minute) needs to be solved with given formula

𝑅𝑃𝑀 = ^60∙𝑓𝑝 2

, (1)

where 𝑓 is the frequency in Hertz and 𝑝 is the number of poles. This means ^𝑝

2 is the number of pole pairs. For example, for a 2-pole generator the needed rotational speed for 50 Hz would be 3000 RPM, when for a 4-pole generator the required rotational speed is 1500 RPM, which is half of the rotational speed of the 2-pole generator. [1]

Electrical machines can be constructed with permanent magnets or electric magnets.

The south and north pole of a permanent magnet are fixed, and the magnetic field exists permanently. An electric magnet is made by passing a current through a conducting wire, which is wrapped around a piece of iron. In the electric magnet, the magnetic field and the magnet polarity are therefore not fixed, and they depend on the direction of the current flow. Electrical machine’s shaft is rotated by changing the current direction when the same poles meet. To help understand this, an illustration of a single-phase rotating magnet is shown in Figure 2. Basically, the current direction is changed once the rotor pole reaches the same pole end of the stator to keep the rotor rotating.

Figure 2. Simple permanent magnet construction.

In three-phase electrical machines, there are three windings instead of one to rotate the rotor. This enables starting and controlling of the machines to be smoother. [1]

The three-phase distributed winding in a three-phase synchronous machine stator is also called an armature winding. Synchronous machine’s rotor has a field winding which is normally fed from an external source or through slip rings or brushes. In salient pole

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machine construction, rotor poles are magnetized with an external electrical supply. Ro- tor poles of a synchronous machine are excited by a DC current, and the stator windings are connected to an AC supply. [1]

In a synchronous generator the field current 𝐼_𝑓 flows through the rotor field winding. The current creates a sinusoidally distributed flux in the air gap. When the machine is rotated by a prime mover, it produces a revolving field in the air gap. The revolving flux will induce voltages in the stator windings. The induced voltages are called excitation voltages 𝐸𝑓. [1]

2.1 Generator construction

Electrical machines are designed considering IEC and IEEE standards. From IEC standards, IEC 60034 applies for rotating electrical machines. From IEEE standards, IEEE C50.13 applies for cylindrical-rotor synchronous generators and IEEE C50.12 applies for salient-pole synchronous generators. [2]

ABB generators have modular structure, which allows various mechanical modifications to the generator design. In Figure 3 is the cross-section of a generator with integrated shaft fan. The fan cools the generator down during an operation to prevent overheating.

From the grid code compatibility point of view, the generator reactance, inertia and the response time play important role in designing the generator. As an example of the mod- ification to achieve the grid code requirements: a flywheel can be added to the generator construction to increase the generator inertia. The additional inertia, in addition to optimal rotor design, helps to achieve the low voltage ride-through requirements of the given grid code. [3]

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Figure 3. Cross-section of a generator with integrated shaft fan. [3]

Another example of possible modifications is that the generator reactance can be adjusted by modifying the stator windings and the magnetic core. The generator reactance depends on the air gap, number of coil turns, winding configuration and length, and other active component dimensions. In Figure 3 the salient pole construction of rotor and the generator stator can be seen around the shaft. When modifying the design of the generator, the performance is also affected, which is why careful optimization of the parameters is important. A cross sectional drawing and a dimension drawing of a generator are presented in Figure 4.

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a) Cross sectional drawing.

b) Dimension drawing.

Figure 4. a) Cross sectional drawing and b) Dimension drawing of a generator.

In Figure 4b is the outline of the generator. The stator and the rotor are covered with a steel frame. The stator is crimped inside the frame and the stator windings are connected

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through the stator frame to the main connection points. Around the shaft ends are bearings. Requirements for bearing type come firstly from the customer but are also affected by the classification requirements and the requirements of the operational area and the preferred lubrication type. In the machine in Figure 4, the bearings are lubricated using separate jack-up systems, which might mean that the machine is expected to have fre- quent starts, or it is expected to be rotated in slow speed. Jack-ups save the bearings not to wear out so fast in case of slow rotational speeds. The shaft of the generator is stationarily fitted into the rotor, and it transfers movement from and to the rotating electrical machine. In case of a generator, a motor shaft, which is connected to the generator shaft through coupling, rotates the shaft of the generator. In addition to main parts, cooling of the generator and instrumentation are important. There are many types of heat exchanger systems available for the machines. In the machine in Figure 4, the cooling is done through the top of the machine with air flow. Air inlet and outlet directions are marked in Figure 4b. Instrumentation includes everything from temperature and vibration detectors mounted into the generator to an additional heater for storage. The instrumentation is typically decided by customer.

ABB synchronous generators have brushless excitation controlled with automatic voltage regulators (AVR) and they are self-excited and therefore do not need additional separate source of excitation power. More information about AVRs is in chapter 3.2.1.

2.2 Equivalent circuit

An equivalent circuit is used to study performance characteristics of synchronous machines. When studying the steady state behaviour of the machine, circuit time constants of field and damper windings do not need to be considered. [1]

A general representation of the equivalent circuit for synchronous machine per phase is presented in Figure 5 including phasor diagrams.

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Figure 5. Equivalent circuit and phasor diagram for (a) synchronous generator and (b) synchronous motor. [1]

For a synchronous generator the equivalent circuit is

𝐸_𝑓 = 𝑉_𝑡+ 𝐼_𝑎𝑅_𝑎+ 𝑗𝐼_𝑎𝑋_𝑠. (2)

Voltage drops 𝐼_𝑎𝑅_𝑎 and 𝑗𝐼_𝑎𝑋_𝑠 are added to terminal voltage 𝑉_𝑡 to obtain excitation voltage 𝐸_𝑓. 𝑋_𝑠 represents synchronous reactance which is a sum of reactance of armature reac- tion or magnetizing reactance 𝑋_𝑎𝑟, and leakage reactance 𝑋_𝑎𝑙. 𝑅_𝑎 represents armature resistance and 𝐼_𝑎 represents armature current, which are both presented in the equivalent circuit and the phasor diagram in Figure 5. For a synchronous motor the equivalent circuit is

𝑉_𝑡 = 𝐸_𝑓+ 𝐼_𝑎𝑅_𝑎+ 𝑗𝐼_𝑎𝑋_𝑠. (3)

In case of a synchronous motor, the voltage drops are subtracted from 𝑉_𝑡 to get 𝐸_𝑓. [1]

For salient pole synchronous machines, DQ modelling is used. In DQ modelling, the synchronous machine characteristics are divided into D- and Q-axis. D-axis refers to the axis going along the salient pole and Q-axis refers to the axis between the poles. Com- pared to the equivalent circuit of the cylindrical pole synchronous machine, flux and magnetomotive force (m.m.f.) are divided between the two poles. Field m.m.f. 𝐹_𝑓 and flux Φ_𝑓 act along the D-axis and armature m.m.f. 𝐹_𝑎 and flux Φ_𝑎 act along the Q-axis. [1]

D- and Q-axis currents 𝐼_𝑑 and 𝐼_𝑞 produce voltage drops 𝑗𝐼_𝑑𝑋_𝑑 and 𝑗𝐼_𝑞𝑋_𝑞, where 𝑋_𝑑 and 𝑋_𝑞 are D- and Q-axis synchronous reactances. The phasor diagrams of salient pole synchronous generator are presented in Figure 6.

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Figure 6. Phasor diagrams of salient pole synchronous machine working in genera-

tor mode. [1]

In Figure 6c armature resistance 𝑅_𝑎 is neglected and phasor 𝐸_𝑓^′ therefore shows excitation voltage if saliency is neglected. Since phasors 𝐸_𝑓 and 𝐸_𝑓^′ are similar, if no high ac- curacy is needed, excitation voltage can be calculated based on phasor 𝐸_𝑓^′ with a following formula

𝐸_𝑓^′ = 𝑉_𝑡+ 𝑗𝐼_𝑎𝑋_𝑑. [1] (4)

Next, more accurate calculating of excitation voltage in salient pole machines is inspected. Synchronous reactances are formed as a sum of armature reactance of the respective axis and armature leakage reactance

𝑋_𝑑= −𝑋_𝑎𝑑+ 𝑋_𝑎𝑙 and (5)

𝑋_𝑞 = 𝑋_𝑎𝑞+ 𝑋_𝑎𝑙. (6)

Leakage reactance is linked to the stator winding and therefore it is same for both 𝑋_𝑑 and 𝑋_𝑞. The D-axis is more important when calculating as the machine usually operates at small pole angle and reluctance is higher along the Q-axis. Therefore, the D-axis is dominant over the Q-axis, 𝑋_𝑑 > 𝑋_𝑞. The equivalent circuit of a synchronous generator (2) can be modified to take into account different axis voltage drops. As presented in the equivalent circuit in Figure 6a, excitation voltage can be calculated with

𝐸_𝑓 = 𝑉_𝑡+ 𝐼_𝑎𝑅_𝑎+ 𝑗𝐼_𝑑𝑋_𝑑+ 𝑗𝐼_𝑞𝑋_𝑞, (7) where armature current 𝐼_𝑎 can be also presented with 𝐼_𝑑 and 𝐼_𝑞 currents

𝐼_𝑎= 𝐼_𝑑+ 𝑗𝐼_𝑞. (8)

From the phasor diagram in Figure 6b we can see that armature current 𝐼_𝑎 is lagging excitation voltage 𝐸_𝑓 by angle 𝜓, which is also called an internal power factor angle.

Since 𝐸_𝑓 goes along the Q-axis, the angle between 𝐼_𝑎 and 𝐼_𝑞 is also the same. Angle 𝜙 between 𝐼_𝑎 and 𝑉_𝑡 is called a terminal power factor angle. The angle relationship is

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𝜓 = 𝜙 + 𝛿, (9)

when the machine is working on a generator mode. If angle 𝜓 or angles 𝜙 and 𝛿, which is the angle between 𝐼_𝑞 and 𝑉_𝑡, are known, currents 𝐼_𝑑 and 𝐼_𝑞 can be obtained from current 𝐼_𝑎:

𝐼_𝑑= 𝐼_𝑎𝑠𝑖𝑛𝜑 = 𝐼_𝑎sin⁡(𝜙 + 𝛿) (10)

𝐼_𝑞= 𝐼_𝑎𝑐𝑜𝑠𝜑 = 𝐼_𝑎cos⁡(𝜙 + 𝛿). [1] (11)

The real power in a salient pole synchronous machine consists of two real power values.

The first real power value is caused by excitation voltage, and it is same for both cylindrical rotor and salient pole rotor. The second real power value, that only applies to salient pole machines, is caused by the salient poles. It takes into account reactances in D- and Q-axis, 𝑋_𝑑 and 𝑋_𝑞. This power produces reluctance torque. Reluctance torque is not dependent on the field excitation and therefore does not exist if 𝑋_𝑑 = 𝑋_𝑞. [1]

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3. GRID CODES

Grid codes are a set of regulations made for the electricity network to ensure reliable and safe functioning of the grid. All connection points to the grid need to fulfil the grid code requirements of the respective TSO (Transmission System Operator). To maintain a stable grid and in order to avoid frequency fluctuations, it is important that the production and the demand of energy are in balance. With fossil fuel-fired big energy production plants, the energy production is even and can be easily adjusted to match the energy consumption of customers. The global energy consumption is increasing all the time.

With the rapidly increasing number of renewables, like wind and solar power plants, the grid has become much more complex, and the energy production has become more uneven. Therefore, the grid stability is even more important. In addition to energy production being unpredictable, also the energy consumption becomes unpredictable with larger scale consumption devices like electric vehicles. [3]

Grid codes include requirements for ranges of frequency variation, ranges of voltage variation, active and reactive power capability, FRT capability and power quality for the connection points to the grid. [2] FRT capability describes how the connection point with- stands faults happening in the grid. Today the connection point is also required to support the grid during and after a fault event. As an example, if a lightning hits a transmission line and causes a voltage spike, which leads to several power generating units being disconnected, it would cause a voltage or frequency drop in the transmission line. The voltage or frequency drop might further lead to other connection points being also disconnected from the grid. The FRT requirements in the grid codes require the units to be able to withstand such faults. The FRT capability requirements are further described in the grid codes of the specific area. The active and reactive power capabilities describe the equipment’s capability to withstand harmonics and voltage and frequency variations in normal operation and in exceptional and disturbance situations. The requirements are determined by respective TSOs, and the connection points needs to be able to fulfil the requirements accordingly. In addition to the requirement to withstand the voltage and frequency variation, the power plant needs to be able to participate in frequency and voltage regulation. [3]

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3.1 Grid codes in different areas

TSO is an authority responsible for local electricity transmission and therefore they have set requirements for all the connection points to the grid. The requirements ensure that all connection points support the grid to work smoothly. [3] In the following chapters different grid code requirements and TSOs are explained in more detail.

3.1.1 ENTSO-E – European grid code

European Network of Transmission System Operators for Electricity, or ENTSO-E in short, has released a common grid code, which applies to roughly all grids in European area. All connection points need to fulfil also ENTSO-E grid code in addition to local TSOs grid codes. ENTSO-E regulations set a framework for TSO grid codes. If a connection point does not fulfil ENTSO-E requirements, the TSO is required to remove the connection from the grid. [4] The European grid code is set to guarantee equal and non- discriminatory conditions for the internal energy markets, to ensure the security of the network, and to unify the connection requirements for all grid codes [5]. The goal of EN- TSO-E grid code is also to support European Union’s energy objectives of cutting green- house gas emissions, having energy savings and increasing the number of renewables.

[4]

Appendix A presents the whole network that is operated by ENTSO-E. The complexity of the transmission system and the connections between the networks of different areas can be seen in the map and it is clear why common regulations are needed for the transmission system network. The different colours in the transmission lines mean different sizes of voltage lines from 750kV to 110kV and DC transmission lines. [4] In Figure 7 we can take a closer look to the transmission lines between Northern European countries.

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Figure 7. Closeup of the transmission system network operated by members of EN- TSO-E in Northern Europe. [4]

In the closeup we can see that there are connecting AC transmission lines on-shore mostly of 300-400 kV and off-shore DC transmission lines between the Northern Euro- pean countries.

ENTSO-E divides power plants into four types from A to D based on their rated capacity and voltage level in the connection point to the grid. The requirements apply to connection points larger than 0.8 kW rated capacity. Type A power plants include all connections under 110 kV connection point voltage and minimum 0.8 kW rated capacity. Type B-D capacity and voltage requirements depend on the area of operation in Europe and how the TSO has determined the connection point types. In ENTSO-E requirements the operating area is divided into Continental Europe, Great Britain, Nordic countries, Ireland and Northern Ireland and Baltic countries. The different grid codes in those areas however may vary. Type A power plant has the least requirements for network connection and operation, and the type D power plant has the most requirements. In addition to the requirements given, type D power plant needs to fulfil also the requirements given for type A-C power plants. [6]

Grid codes in Finland, Italy and Germany are explained in more detail in the thesis as they are common countries for grid code projects in ABB. The base of regulations for all TSOs is the ENTSO-E grid code (2016/631 Requirements for Generators [6]). Each TSO have then added their own national additions and specifications to the grid code. When inspecting the grid codes from equipment supplier point of view, it is important to notice that in most cases the given grid code requirements are determined for the connection point to the grid and not for a single equipment in the system like a generator. [6]

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3.1.2 Grid code requirements in Finland

In Finland the electricity transmission system operator is Fingrid. The latest grid code specifications for operational performance of power generating facilities from Fingrid is VJV2018 specification [5]. VJV2018 specifications for power plants connected to the grid apply to all power plants larger than 0.8 kW power. The power plants with higher power ratings are divided into four groups based on the size of the power plant similarly to ENTSO-E grid code for Nordic countries. [5] In VJV2018 the type B-D plants are divided based on the voltage of the connection point to the network and the capacity ratings as follows: type B has the connection point voltage under 110 kV and the rated capacity over 1.5 MW, type C has the connection point voltage under 110 kV and the rated capacity over 10 MW, and type D has the connection point voltage under 110kV and the rated capacity over 30 MW or the connection point voltage is over 110 kV despite the rated capacity. [6] As an example from the grid code, VJV2018 has determined voltage and frequency variation requirements for the connected power plants. In the rated connection point of 110 kV, the normal voltage variation range is determined to be 105-123 kV and in exceptional and disturbance situations the voltage range is 100-123 kV. The rated frequency in Nordic electricity network is 50 Hz and the frequency variation in normal operation is 49.0-51.0 Hz and in exceptional and disturbance situations the variation might be even 47.5-51.5 Hz. Type A power plant needs to be able to operate under normal frequency variation and withstand the frequency variation in exceptional situation for at least 30 minutes. [5]

The frequency variation requirements for type D power plants in different ENTSO-E areas are given in Figure 8.

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Figure 8. The requirements for frequency variation in different ENTSO-E operating areas. [7]

From the figure we can see that for example in Nordic area type D power plant is required to withstand the frequency variation between 51–51.5 Hz and between 47.5-48.5 Hz for 30 minutes [7].

The VJV2018 requirement for FRT capability is given for type B-D power plants. The FRT requirement applies to 3-phase and 2-phase short circuit events and 2-phase and 1-phase earth short circuit events. A synchronous power plant needs to withstand faults in the grid without losing the synchronous operation. [5] Figure 9 explains the FRT requirement for fault event after which the synchronous power plant needs to continue to operate normally.

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Figure 9. The FRT capability requirement graph for a synchronous power plant. [5]

The fault event lasts for 150 ms and during that time the voltage drops to 0.05 p.u. The voltage level is 1 p.u. before the fault event. Even if several voltage disturbances happen, the power plant must stay connected to the grid. During a voltage disturbance event, the power plant needs to restore the active power that was before the disturbance event in 1-3 seconds from the start of the event. [5]

3.1.3 Grid code requirements in Italy

For the countries in Continental Europe, type B-D power plants in ENTSO-E are divided based on the ratings as follows: type B has the connection point voltage under 110 kV and the rated capacity over 1 MW, type C has the connection point voltage under 110 kV and the rated capacity over 50 MW, and type D has the connection point voltage under 110kV and the rated capacity over 75 MW or the connection point voltage is over 110 kV despite the rated capacity. [6]

In Italy the electricity transmission system operator is Terna. Terna’s applicable grid code for new power plants connected to the national grid is included in the grid codes chapter 1 section 1C [8]. In Terna grid codes the requirements are divided based on generator groups which can be considered as power plant types. The groups are divided to type B-D as follows: generator group type B has the connection point voltage under 110 kV and the rated capacity over 11.08 kW, type C has the connection point voltage under 110 kV and the rated capacity over 6 MW, and type D has the connection point voltage under 110 kV and the rated capacity over 10 MW or the connection point voltage over

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110 kV despite the rated capacity. For Terna grid code the frequency variation requirement is same as in Fingrid grid code with normal frequency variation of 47.5-51.5 Hz.

More specifically, the power plant needs to stay connected minimum of 4 second in lower frequencies between 46.5-47.5 Hz and minimum 1 second in higher frequencies between 51.5-52.5 Hz. For frequencies higher and lower than previous limits, the power plant needs to remain connected minimum 0.1 seconds. For under 110 kV systems the grid code requirement is to withstand voltage variation from 85% to 115% of the nominal voltage. [8]

The requirements for FRT capability in Terna regulations are determined for type C and D synchronous generator groups in symmetrical and asymmetrical faults. For type D synchronous generator group, the FRT capability requirement in symmetrical faults is according to Figure 10.

Figure 10. The FRT capability requirement graph for type D synchronous generator group for symmetrical faults. [8]

The blue line in the figure is an FRT profile. The FRT profile represents the zone where the generator group is not permitted to be disconnected. Under the FRT profile is the zone where the generator group is permitted to be disconnected. The FRT profile is checked for each short-circuit-power value and the criterion used is compliance with applicable technical standards. The activation of disconnection logic happens during faults when the permitted voltage dips are exceeded. The voltage dips are determined in terms of depth and duration as per the graph in Figure 10. [8]

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3.1.4 Grid code requirements in Germany

In Germany, VDE FNN is the technical regulator of power grids. VDE FNN sets the grid codes for the transmission system operators and users. VDE FNN has over 450 compa- nies and organizations as their members, which are working for the power grid including network operators, power generating plants operators, manufacturers etc. VDE-AR-N 4120 is a VDE FNN grid code, which applies to high voltage connections. High voltage connections have nominal connection voltage over 110 kV and therefore the grid code applies to ENTSO-E type D power plants. VDE-AR-N 4110 is a VDE FNN grid code, which applies to medium voltage connections. Medium voltage connections have nominal connection point voltage between 1kV to 60 kV and therefore VDE-AR-N 4110 applies to ENTSO-E type B-C power plants. [9] VDE FNN has divided the power plants into two types: type 1 includes directly linked synchronous generators and type 2 includes all the generators that do not comply with type 1. In VDE-AR-N 4110 requirements the same frequency variation limits apply to the power plant connections as in Fingrid and Terna grid codes. The normal voltage variation that the connection point needs to withstand is between 90% and 110% of the nominal voltage. [10]

The FRT capability requirement in VDE-AR-N 4110 grid code in symmetrical and asymmetrical faults for type 1 generator group is according to Figure 11.

Figure 11. The FRT capability requirement graph for synchronous generator group.

[10]

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The variable 𝑈_𝑁𝐴𝑃 in the graph vertical axis is the RMS value of the actual connection point voltage. In the graph the blue line marks the upper FRT limit curve, the red line marks the lower FRT limit curve for 3-phase faults, and the green line marks the lower FRT limit curve for 2-phase faults. [10]

3.2 What grid codes mean for generator manufacturer

Grid codes have requirements for connection points to the power grids. This means that the responsibility of the connection point fulfilling grid codes is on the connecting operator. ABB, as a generator and AVR manufacturer, supports the customers and operators to fulfil the requirements of the grid codes. It is important to notice that the grid codes do not directly give regulations for the generators, but for the connection point to the grid.

IEC and NEMA design standards affect machine design. However, these machine design standards do not take into account the continuously updating grid code requirements. This means that when the machine is designed based on design standards, it does not necessarily fulfil grid code’s requirements, and therefore requires additional studies and product development. [11] Modifications to ABB generators have already been made due to different grid code requirements for different projects.

The grid code requirements for type D power plants discussed earlier for Finland, Italy and Germany are presented in Table 1.

Simplified table of the grid code requirements. [5, 8, 10]

The table summarizes the grid code requirements for FRT and voltage and frequency variation in the three different grid codes discussed in chapters 3.1.2, 3.1.3 and 3.1.4.

The frequency and voltage variation requirements set by grid codes may exceed the limits of the operating voltage and frequency of the design standards of the generator.

That causes risks of compromising the reliability, stability, and the performance of the generator, if machine design is not properly modified. An example comparison of IEC 60034-3 design standard’s voltage and frequency requirements with Swedish TSO’s

Grid code require- ment

VJV2018 (Fingrid) Terna VDE-AR-N 4110 (VDE FNN)

FRT 150 ms 200 ms 150 ms

Voltage variation for 110 kV connection point voltage

Normal 105–123 kV / Change 100–123 kV

Normal 85% - 115%

of the nominal voltage (For 110kV connection 93.5- 126.5 kV)

Normal 90% - 110% of the nominal voltage (For 110kV connection 99- 121 kV)

Frequency variation for 50 Hz grid fre- quency

Normal 49.0–51.0 Hz / Change 47.5–

51.5 Hz

Normal 49.0–51.0 Hz / Change 47.5–

51.5 Hz

Normal 49.0–51.0 Hz / Change 47.5–51.5 Hz

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Svenska krafnät grid code (SvKFS) requirement is presented in Figure 12. [11] IEC 60034-3 is a design standard that applies to large three-phase synchronous generators [12].

Figure 12. Comparison of IEC 60034-3 requirement of generator voltage-frequency limits with grid requirements of SvKFS. [11]

The Swedish SvKFS voltage-frequency curve is given in dark blue in the figure. IEC voltage-frequency curves are given in red and light blue. From the curves we can see that SvKFS requires the connection point to withstand larger variations than IEC does.

Comparing the voltage variation requirements in machine design standard and grid code is not as straight forward. Grid codes tend to specify the limits of voltage variation requirements for the connection point to the grid and not for a single component. Generator design standards specify voltage variation limits for the generator. [11]

A simplified connection drawing of a generator terminal voltage and the grid voltage can be seen in Figure 13.

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Figure 13. Simplified generator-grid connection diagram. [11]

Grid codes often specify voltage ranges for the grid connection point and the generator design standards specify the requirements for the generator terminals point. In the connection diagram the generator terminals and the grid connection point are separated with a transformer. The relationship between the generator terminal voltage and the grid voltage on the transformer high voltage side are affected by active power, power factor and transformer reactance. Therefore, they need to be considered when comparing grid code requirements to generator parameters. [11]

Before the equipment is taken into use, grid code compliance must be checked with variety of tests and simulations. Automatic voltage regulators (AVR) also play an important role in fulfilling grid code requirements.

3.2.1 Automatic voltage regulator

AVR is used for synchronous generators and motors to achieve constant output voltage even with varying load. It controls the output voltage by changing generator excitation voltage and therefore generator excitation current. AVRs are used in all synchronous machines except in small self-regulating units. AVRs are powered by different excitation power sources. [13]

There are analog and digital AVRs available, but mostly digital AVRs are used. Digital AVR has better controllability and faster response time than analog AVR and therefore it is more suitable for grid code applications. Also, optimal tuning in digital AVR allows to enhance the power quality. [14]

Proportional integral derivative (PID) controller has three control parameters: proportional, integral and derivative gains and it is often most preferred one for AVRs. The PID controller is used for the control point to achieve a given setpoint. A P-controller’s ma- nipulated variable is proportional to the recorded system deviation. The P-controller han- dles the immediate error in the system. A PI-controller is formed connecting the P-controller and an I-controller in parallel. The P-controller’s main disadvantage is a continuous deviation, meaning that the setpoint is never completely attained and this causes steady-

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state error. The I-controller is used to correct the system deviations, meaning the PI- controller does not have permanent system deviation. The PI-controller is, when correctly set, stable and fast. When a D-controller is added to the PI-controller, it becomes the PID-controller, which reaches its setpoint and steady state faster, when the gains are correctly set. The D-controller has a faster response than the P-controller due to its ma- nipulated variable is generated from the rate of change in the system deviation, rather than from its amplitude, which is used by the P-controller. However, unlike the P-controller, the D-controller cannot usually be used by itself due to its inability to detect permanent deviations. Due to the fast response time, the PID-controller is suitable for grid code requirements. [15] In Figure 14 is presented a PID-controller block in IEEE standard.

Figure 14. PID-controller a) block diagram and b) implementation. [16]

The PID-controller diagram includes three gains: proportional gain 𝐾_𝑝, integral gain 𝐾_𝐼 and derivative gain 𝐾_𝐷. The PID-controller transfer function is formed from these three gains

𝐾(𝑠) = 𝐾_𝑃+^𝐾^𝐼

𝑠 + ^𝑠𝐾^𝑠

1+𝑠𝑇_𝐷, (12)

where 𝑇_𝐷 is a lag time constant. [16]

Some AVR functionalities that support grid code compliance are a PF/VAR controller with low voltage FRT and over voltage FRT support function and a Power System Sta- bilizer (PSS). The PF/VAR control structure supports network dips by stabilizing the network. The dynamic detection of FRT allows to regulate the set point dynamically, which avoids heavy reactive power overshoot to the network. The PSS improves the stability of the generator operation range. [14]

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3.3 Challenges with design and handling process

A generator manufacturer should be sure that the generator provided fulfils the customer requirements including the given grid code requirements. The manufacturer is expected to have fast response time especially in the beginning of new and potentially new projects. In most cases generator parameters and possible other information related to fulfilling grid code requirements are needed to be asked from the back-end teams, which takes time. The delays in customer responses compromise possible projects and cause a long work queue for the back-end teams. This thesis wants to speed up this process and aims to create simulation models, documentation and directions to support the front- end users for faster response times.

The goal of the development work is to create easy enough simulation models for the front-end users to use before requesting assistance from the back-end (like support and R&D). In simple projects, the salesperson and the customer could check the project and grid code requirements by themselves and when new and more difficult projects come, where more complex calculations and simulations are needed, they could ask help from the back-end. Grid codes give demands to generator design. Sometimes generators that are not off-the-shelf products are needed to fulfill the grid code requirements. Electrical machine parameters such as SCR (short circuit ratio), reactances and inertia must be carefully designed through extensive simulations and factory acceptance tests.

Another goal of the development work of the handling processes would be to ease the work of the sales support by reducing the requests of assistance sent to the back-end.

Asking assistance from different departments takes time and causes additional work.

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4. SIMULATIONS

ABB has developed simulations to check the grid code compliance of customer’s power plants. These simulations, modellings and analysis are done for all ordered grid code projects. When the project is in the pre-sales phase and early ordering phase, some verification calculations and simulations are needed to find the correct machine design and the needed parameters for the additional equipment like AVRs. These simulations and calculations are currently done by R&D with Mathworks Simulink. The existing Sim- ulink model for FRT is briefly discussed in chapter 4.5.

The simulations created in the thesis are done with DIgSILENT simulation program called PowerFactory 2021 release. In the thesis, three simulation models were created:

model for FRT simulation, load connection and rejection simulation and load flow simulation. The simulation models are explained in more detail in the following chapters. A general view of PowerFactory simulation program is presented in Figure 15.

Figure 15. A general view of PowerFactory simulation interface.

The interface includes three windows aligned to the same view: two windows, which are side by side, are used now as the simulation model window on the left and the signals on the right. The third window is the output window showing results for simulations and scripts. On the left is presented the drawing tools in the object library of PowerFactory.

In each simulation model, the generator parameters need to be given for the simulation generator object. For this, a script which transfers the parameters automatically from a

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selected Excel worksheet to the generator object, was also created and it is explained in more detail in chapter 4.4.

Synchronous generator and general load schematics are presented in Figure 16.

a) b)

Figure 16. PowerFactory schematics of a) a synchronous generator and b) a gen- eral load.

The component schematics are copied from DIgSILENT library, but the user should set the needed parameters for all the components that are used in the simulation. In these simulation models the following components are used: a synchronous generator, a general load, an external grid, two bus bars and a transformer. The required parameters are explained in more detail later in chapter 4.4.

A simplified AVR model was used in the simulations: a common model of general AVR called AC7B is presented in Figure 17. The AVR model and a governor model TGOV2 are added for the simulation from DIgSILENT library under PSS/E compatible library.

Figure 17. A common model AVR used in the simulation.

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The parameters for AVR were collected from IEEE 421 standards [16] and a data sheet from an example project. The same simulation objects and AVR parameters were used in all the simulations in chapters 4.1, 4.2 and 4.3.

In general, the simulation models consist of the synchronous generator, which is connected to bus bar 1. Bus bar 1 voltage is 11 kV. Bus bar 1 and Bus bar 2 are connected to each other through the transformer. Bus bar 2 voltage is 20 kV. The external grid is also connected to bus bar 2. These objects and their arrangement can be seen in Figure 15 FRT simulation model. For the load connection and rejection and the load flow simulation models, the general load is also connected to bus bar 2. The created simulation models are explained in more detail in each simulation chapter.

4.1 FRT simulation

During the FRT simulation, the generator behaviour is simulated during a fault event.

The simulation model resembles the one in Figure 18, where the generator is connected to the external grid through the transformer.

Figure 18. Simulation model for FRT simulation.

When running the FRT simulation in PowerFactory, the event setting is according to Fig- ure 19. Fault events, where they happen, and how long they last are highlighted in yellow

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in the figure. The fault events are listed under the column “Name”. The object, where the fault is set to happen, is listed under the column “Object”. The times, when the events are set to happen, are listed under the column “Absolute s”.

Figure 19. Fault event selection for FRT simulation.

During the FRT simulation a 3-phase short-circuit is set to happen on bus bar 2. This represents a short-circuit event happening in the grid. An event called ‘3-phase Short- Circuit Event’ sets the short-circuit to happen at 4 s. An event called ‘Clear Short-Circuit’

clears the short-circuit at 4.15 s.

The example simulation is run for 15 s and the simulation signals are presented in Figure 20.

Figure 20. Simulation signals for FRT simulation without power flow to the grid.

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The resulting signals include from top to bottom: generator line voltage in p.u. value, generator current in p.u. value, generator active power in MW and excitation voltage and excitation current of the generator in p.u. values. Per unit (p.u.) values are used in the simulation results excluding the power signals. The p.u. values are used, because they simplify the comparison of the changing signals to the rated values. MW and MVAr units are used for power signals as the p.u. values are not available. The per unit calculation method is briefly explained in Appendix B. Positive-sequence voltage and current in p.u.

values are available for inspecting in PowerFactory. An ideal unloaded generator should only produce positive-sequence voltages. Positive-sequence values consist of balanced three-phase voltage and currents. [17]

The short circuit lasts for 0.15 s in the simulation. During the short-circuit event the generator voltage dips close to zero. The generator current peaks close to 4 p.u. and then settles back down close to zero. Generators are designed to withstand a limited number of short-circuits. Generator’s protection can be tested for short-circuits, but the tests should be limited to minimum possible amount. This is because each short-circuit short- ens the age of the generator and the connected equipment. High current causes also temperature rise in the generator stator which might cause reduction in the lifetime of generator components or even damage the generator.

The generator active power first dips to around -1 MW during the short-circuit and then peaks to around 0.5 MW. The active power starts to oscillate after the clearance of the short-circuit before settling back down. During this transient period in real life situation, the governor controller will activate to reduce the mechanical power from the engine.

The excitation voltage dips to zero at the time of the short-circuit. Once the short-circuit is cleared, the excitation voltage rises to 1.4 p.u. and then settles back to 1 p.u. The excitation current peaks close to 4 p.u. at the time of the short-circuit and then comes back down to 1 p.u.

The simulation in Figure 20 is run without any power flow from the generator to the grid.

Because of this, the generator active power in simulation results is 0 MW in normal operation. If we add power flow from the generator to the grid, we get the signals in Figure 21.

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Figure 21. Simulation signals for FRT simulation with power flow to the grid.

Power flow of -13 MW and -8 MVAr is added to the grid from the generator. The negative power flow to the grid means that the grid is consuming the power. The generator is about 63% loaded in the example simulation, but it could be selected case by case. From the results in Figure 21 we can see that the generator current is now 0.63 p.u. in the normal operation and the generator active power is now 13 MW in the beginning of the simulation. We can also see some changes in the other signals. The oscillation in the simulated values can be affected by changing the value of the power flow to the grid. As soon as the generator voltage drops at the time of the short-circuit, the AVR is respond- ing quickly and regulating the voltage by pushing high excitation current to bring the generator voltage back to steady state.

In the load connection and rejection simulation and the load flow simulation the power flow to the grid is kept as 0 MW and 0 MVAr.

4.2 Load connection and rejection simulation

During the load connection and rejection simulation, the load behaviour is simulated over a certain period. Changing loads cause a risk of falling out of synchronism for synchronous machines. This is not permissible according to grid code and naturally undesirable

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for the operator. The simulation model created for the load connection and rejection simulation is presented in Figure 22.

Figure 22. Simulation model for load connection and rejection simulation.

In addition to the model for FRT simulation, the general load is included on bus bar 2 and the external grid is removed from the model. The goal of the load connection and rejection simulation is to determine the performance class of the machine. The operation limit values for electrical machines are determined by IEC 60034 standard [18].

During the load connection and rejection simulation, load behaviour is simulated over a certain period. In the test simulation, the following simulation events were added: first the load is turned off at 4 s and then on at 70 s. The simulation time in the test is 120 s.

The load active and reactive powers and the generator reactive power were added to the simulation signals, which are presented in Figure 23.

Development of grid code handling process and simulation models for synchronous machines

Tiia Moilanen