Control Strategies of MMC-HVDC Connected to Large Offshore Wind

Control strategies of MMC-HVDC connected to large oﬀshore wind farms for improving fault ride-through capability

A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY

Woojung Choi

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science in Electrical and Computer Engineering

Professor Ned Mohan

I would like to express my gratitude to the many people who helped me complete my studies during my graduate school days. Most of all, I am extremely grateful to Professor Mohan, whose perceptive criticism, kind encouragement, and willing assistance helped bring my research to a successful conclusion. I would also like to thank Professor Jungwon Choi and Peter Kang for reviewing my thesis.

i Dedication

I dedicate my thesis to my family and many friends. Above all, I would like to dedicate my thesis to my beloved parents, whom I respect the most. I also dedicate this to my wife, Jisuk, my two daughters, Yubeen and Bogyeong, who have endured sometimes hard and diﬃcult times for two years. In addition, I would like to express my sincere gratitude to Pastor Baik’s couple for their great help to our family in the United States.

ii Abstract

This paper proposes strategies to improve fault ride-through (FRT) capability of the modular multi-level converter (MMC) - high voltage direct current (HVDC) system connected to large oﬀshore wind farms and performs simulations. In oﬀshore wind power plants, HVDC system is indispensable for long-distance high-capacity transmission. The voltage rise of HVDC-link happens inevitably due to energy accumulation to satisfy low voltage ride-through (LVRT) regulation when a main grid fault occurs. This paper presents strategies for controlling HVDC-link voltages while minimizing the application of DC choppers and the mechanical and electrical stress of wind turbines through fast fault detection and current limit control of the master controller and wind turbine converter. PSCAD/EMTDC simulation is performed to verify the control strategies, and the results show that the FRT capability is enhanced by controlling HVDC-link voltage properly.

iii Contents

Acknowledgements i

Dedication ii

Abstract iii

List of Tables vii

List of Figures viii

1 Introduction 1 1.1 Background and Motivation ...... 1 1.2 Thesis organization ...... 3

2 Offshore Wind Power Systems 4 2.1 General trend of Offshore wind energy ...... 4 2.2 The Configuration of Offshore wind power systems ...... 7 2.2.1 The reason for applying HVDC system to offshore wind farms . . 7 2.3 Wind turbines ...... 8 2.3.1 Type-1 : The fixed speed turbine systems ...... 9 2.3.2 Type-2 : The limited variable speed turbine systems ...... 9 2.3.3 Type-3 : variable speed with partial-scale turbine ...... 10 2.3.4 Type-4 : variable speed with full-scale turbine ...... 11 2.3.5 Wind turbine control system ...... 11 2.3.6 Offshore AC grid ...... 14

iv 2.3.7 VSC-HVDC converters : oﬀshore and onshore station ...... 14 2.3.8 HVDC Cable ...... 15

3 HVDC system for offshore wind farms 16 3.1 Voltage sourced converter for HVDC ...... 16 3.2 VSC converter configurations ...... 17 3.2.1 2–Level converter ...... 17 3.2.2 3–Level converter ...... 17 3.2.3 Modular Multi-level Converter (MMC) ...... 18 3.2.3.1 Operation principle of MMC ...... 19 3.3 MMC-HVDC controls ...... 22 3.3.1 Current controller ...... 22 3.3.2 DC-link voltage control of MMC-HVDC ...... 23 3.3.3 Concept of HVDC system controls connected to wind farms . . . 25 3.3.3.1 Onshore Grid Side Converter ...... 26 3.3.3.2 Offshore Grid Side Converter ...... 26

4 Low Voltage Ride-Through (LVRT) 28 4.1 The Overview of grid codes ...... 28 4.2 LVRT requirements during grid faults ...... 29

5 HVDC-link over-voltage control strategies 32 5.1 The reason for over-voltage of the HVDC-link ...... 32 5.2 The Existing over-voltage control methods ...... 33 5.2.1 The application of a DC chopper ...... 33 5.2.2 The wind power output control based on communication lines . . 35 5.3 The proposed control method ...... 37 5.3.1 Fast fault detection ...... 37 5.3.2 DC-link current control in the main controller ...... 39 5.3.3 Wind power control by current droop ...... 41 5.3.4 Cooperative control with existing method ...... 43

v 6 Simulation 45 6.1 Simulation study ...... 45 6.2 Validation of case studies ...... 46 6.2.1 Normal Operation ...... 47 6.2.2 Case Studies ...... 47 6.3 Simulation Results ...... 49 6.3.1 Case 1: Natural response(without any control method) ...... 49 6.3.2 Case 2: Existing Wind Power Output control ...... 53 6.3.3 Case 3: Application of a DC chopper ...... 56 6.3.4 Case 4: Proposed method ...... 58

7 Conclusions and Future Work 64 7.1 Conclusions ...... 64 7.2 Future Work ...... 65

References 66

vi List of Tables

4.1 Fault ride-through capability for wind turbines in various grid codes . . 31 6.1 Parameters of MMC–HVDC System Model ...... 46 6.2 Simulation cases ...... 48 6.3 Transient simulation scenario ...... 48

vii List of Figures

2.1 Renewable energy share of global electricity production 2018 ...... 4 2.2 Share of electricity generation from variable renewable energy 2018 . . . 5 2.3 Levelized Cost of Electricity(LCOE) of Onshore and Offshore ...... 6 2.4 Capacity of Offshore Wind Power ...... 6 2.5 The typical configuration diagram of offshore wind farm ...... 7 2.6 The configuration of HVDC connection ...... 8 2.7 Type1-SCIG a fixed-speed wind turbine ...... 9 2.8 Type2-WRIG variable speed wind turbine ...... 10 2.9 Type3- DFIG wind turbine ...... 11 2.10 Type4-PMSG a fully rated converter-connected wind turbine ...... 12 2.11 Wind turbines connected to grid via HVDC ...... 13 2.12 Control diagram of offshore wind farms ...... 13 2.13 Control structure for point to point HVDC connecting an offshore wind farm ...... 15 3.1 Scheme of a 2-level VSC ...... 18 3.2 Scheme of a 3-level VSC ...... 19 3.3 Structure of MMC HVDC ...... 20 3.4 Output waveform generated from 11-level converter with 5 DC sources . 21 3.5 Output waveform of MMC ...... 21 3.6 Control block diagram of MMC-HVDC ...... 23 3.7 Control block diagram of a current-controller ...... 24 3.8 Electric equivalent circuit of DC-link ...... 24 3.9 The control conceptual diagram of Onshore grid side converter . . . . . 27 3.10 The control conceptual diagram of Offshore grid side converter . . . . . 27

viii 4.1 Main parameters of FRT requirement ...... 30 4.2 LVRT requirements of various grid codes ...... 31 5.1 The power flow at the DC-link dynamics during a AC main grid fault . 34 5.2 Basic diagram of DC Chopper ...... 34 5.3 Block diagram of control scheme ...... 35 5.4 The diagram of the offshore wind power output control ...... 36 5.5 The flowchart of the wind power output control ...... 36 5.6 The control scheme of proposed method ...... 37 5.7 AC main grid voltage during a fault ...... 38 5.8 Offshore grid AC voltage during a fault ...... 39 5.9 New fault detection algorithm ...... 39 5.10 The current control scheme in main controller ...... 40 5.11 The structure of current limiting function ...... 41 5.12 The basic overview of MMC control system ...... 41 5.13 Analysis of d-axis current during 1-phase fault ...... 41 5.14 Control scheme on wind farm side converter ...... 42 5.15 Analysis of d-axis current during 1-phase fault at the wind farm side . . 43 5.16 Flowchart of the proposed control method ...... 44 6.1 The physical simulation model ...... 45 6.2 The simulation model in PSCAD/EMTDC ...... 46 6.3 Simulation Result during Normal operation ...... 47 6.4 Simulation model-single phase fault ...... 48 6.5 AC main grid voltage during transient analysis ...... 49 6.6 HVDC-link DC voltage during transient analysis ...... 50 6.7 Wind power status during transient analysis ...... 51 6.8 AC main grid Power during transient analysis ...... 51 6.9 Physical Model of Wind Turbine Side ...... 52 6.10 Comparison of in&output power in wind turbine converter during 1φ fault . . 52 6.11 Machine speed and electric torque in wind generator during 1φ fault . . 52 6.12 The HVDC-link voltage via wind power reduction - 1φ fault ...... 53 6.13 The HVDC-link voltage via wind power reduction - 3φ fault ...... 54 6.14 Wind generator side output power-Case 1 and Case 2 during 1-phase fault . . 54

ix 6.15 Grid side output power - Case 1 and Case 2 during 1-phase fault ...... 55 6.16 Machine speed in wind generator - Case 1 and Case 2 during 1-phase fault . . 55 6.17 Electrical torque in wind generator - Case 1 and Case 2 during 1-phase fault . 56 6.18 The HVDC-link voltage via DC Chopper - 1-phase fault ...... 57 6.19 The HVDC-link voltage via DC Chopper - 3-phase fault ...... 57 6.20 Chopper circuit to dissipate excessive power ...... 58 6.21 The DC-link voltage via Proposed method - 1-phase fault ...... 59 6.23 The DC-link voltage via Proposed method - 3-phase fault ...... 59 6.22 The comprehensive analysis of DC-link voltage in 4 cases - 1-phase fault 60 6.24 The comprehensive analysis of DC-link voltage in 4 cases - 3-phase fault 60 6.25 Comparison of output power in wind turbine during 1-phase fault ...... 61 6.26 Machine speed and Electric torque in wind generator during 1-phase fault . . . 61 6.27 The comparison of wind generator side output power in 3 cases - 1-phase fault 62 6.28 The comparison of grid side output power in 3 cases - 1-phase fault ...... 62 6.29 The Comparison of Wind generator’s Machine speed in 3 cases - 1-phase fault 63 6.30 The Comparison of Wind generator’s Electric Torque in 3 cases - 1-phase fault 63

x Chapter 1

Introduction

1.1 Background and Motivation

Green power supply and consumption has become a vital concern for all countries of the world, and renewable energy is emerging as the strongest solution. As a result, countries are increasingly investing in clean energy. Wind power has become one of the most notable renewable energy sources of green power technology as a representative of clean energy in terms of technology and economics. This trend is expected to become clearer in the future. As of 2018, 5.5% of global electricity production was supplied by wind power, and is expected to reach nearly 20% by 2030 [1]. Wind power is largely divided into onshore and offshore wind energy. In the early days, the wind power facilities have been increased mainly by onshore wind power. Offshore wind power has grown since 2010, with an annual capacity growth of more than 30% and global capacity of 23GW in 2018 [2]. Although growth has been limited due to technical difficulties and high investment costs compared to the onshore wind power systems, the demand for offshore wind power capable of stable output with a constant wind speed is steadily growing as construction sites of onshore wind farms are depleted and civil complaints increase due to the noise from the blades of wind turbines. In addition, the gap in the cost of power generation compared to onshore wind power has decreased, and the technology of large-capacity wind power generators is developing. Accordingly, offshore wind power that is capable of producing large-capacity electricity is expected to gradually become a significant renewable energy sources [2].

1 2 For the case of offshore wind farms, long-distance transmission through submarine cables is essential to obtain stable wind resources from as close as tens of kilometers to as far as hundreds of kilometers. When AC power is transmitted over long distances from offshore to onshore, long distance power transmission is impossible without reactive power compensation devices due to the rapid increase in the capacitance of AC cables. As an alternative, the high voltage direct current (HVDC) system, which is free from these problems and is advantageous for long-distance high-capacity transmission, has emerged as a far-reaching facility for grid connection of offshore wind farms. In particular, the voltage sourced converter (VSC) based HVDC system, which enables independent control of active and reactive power with the development of power electronics technology, presents a more attractive advantage as a control system with increasing demand for renewable energy. Recently, the VSC-HVDC system based on modular multilevel converter (MMC), which is configured by connecting modules in series, has been applied. As a result, the technology is rapidly growing to a level that can be applied to the GW level as well as improve the efficiency in the HVDC system [3]. Since offshore wind power with a large capacity has a considerable impact on the grid, grid system stability is guaranteed through the grid code, a grid system-linked regulation, in many countries [4]. In the grid code, there is a low voltage ride-through (LVRT) regulation that maintains the grid connection with the wind farm for a certain amount of time in case of a fault in the main grid system. That regulation depends on the voltage drop rate and the duration of the fault in order to retain system stability. This means that wind farms must continue to power the grid, even under certain fault conditions. However, as the voltage of the AC main grid decreases, the HVDC-link has difficulty transmitting the power generated from the wind farm to the main grid [5]. Without proper actions, system stability can be seriously affected by increasing the harmonic content or interrupting transmission lines for over-voltage protection. In addition, a significant increase in DC voltage may cause over-voltage application of the associated facility and, in particular, potentially affect the life of the DC cable or cause permanent damage [6]. Accordingly, it is necessary to establish a system that satisfies the LVRT regulations and protects the HVDC system by suppressing the DC voltage rise and enables stable operation after the fault is cleared. In order to overcome this problem, various studies have been conducted to suppress the DC voltage rise of the 3 HVDC system in the event of a grid fault [7][8][9][10][11][12]. These studies related to fault ride-through (FRT) suggest strategies in two main ways. One method is to use a DC chopper circuit to quickly dissipate the accumulated energy through a chopper resistor to stabilize the DC-link voltage. Although it is applied in the most powerful and efficient way, it is necessary to find a solution to the increasing costs of installation of cooling equipment as well as the facility, and to the problems of large fluctuation of current and voltage causing stress in the system [7]. Another ap- proach is to reduce the active power delivered to the HVDC-link through wind turbine control. The active power is reduced by adjusting the frequency, voltage or reference power of the wind farm grid using the communication line between converter stations. This requires reliable communication lines and potential communication delays due to long distances between DC-links [10][12]. It also has mechanical and electrical stress problems on the wind turbines as a challenge [6]. In this paper, an algorithm is presented to improve the problems that may arise in the existing methods applied in the event of the main grid fault and to hinder a DC voltage rise in HVDC. To evaluate the performance of the proposed method, the MMC-HVDC link connected to the 900MW large wind farm is simulated in PSCAD/EMTDC, a transient analysis program.

1.2 Thesis organization

This paper proposes strategies of improving fault ride-through capability by controlling DC-link voltage rise of the MMC-HVDC system connected to large offshore wind farms when a fault occurs in the main grid system. This paper includes the following sections. Chapter 2 presents the facilities that make up the power plants, including a description of the type of wind turbines applied to the offshore wind farms. Chapter 3 describes the basic theory and control methods for the MMC-VSC HVDC system. Chapter 4 provides an overview of the LVRT regulations and the regulations of different countries. Chapter 5 presents an explanation of the existing methods and new proposed algorithms for keeping the DC voltage of the HVDC-link stable. In Chapter 6, to verify the control strategy, the simulation is performed using PSCAD/EMTDC and the results are analyzed. Finally, Chapter 7 summarizes the results of this paper and provides suggestions for directions for future research on this topic. Chapter 2

Oﬀshore Wind Power Systems

2.1 General trend of Oﬀshore wind energy

The share of renewable energy sources in global electricity production is increasing slowly but surely. Fig.2.1 illustrates Renewable energy share of global electricity production in 2018. Wind power accounts for the largest portion of electricity production except for hydro electric power, a traditional renewable energy source, and has achieved the fastest technological advancements among the current renewable energy sources. As shown in Fig.2.2, even in terms of the proportion of electricity production in each country, the share of wind power generation is considerably higher than that of photovoltaic power generation.

73.8% Non-renewable electricity Renewable electricity

% Wind power 15.8% 5.5 26.2% Hydropower Renewable electricity 2.4% Solar PV

2.2% Bio-power

Geothermal, CSP 0.4% and ocean power

Figure 2.1: Renewable Energy Share of Global Electricity Production 2018 [1]

4 5 Share of total generation (%) 60 Solar PV

50 Wind power

0 United Denmark Uruguay Ireland Germany Portugal Spain Greece Honduras Nicaragua Kingdom

Figure 2.2: share of electricity generation from variable renewable energy 2018 [1]

Wind power is divided into onshore and offshore wind power plants depending on the location of the wind generators. Onshore wind power, which has built the wind farms on land, has been developed with a focus on lowering costs, mainly in the 2-3[MW] class. As the demand for energy increases, the capacity of wind power generation has increased, but it has been difficult to select sites for onshore wind power generation due to problems related to natural damage and noise. The offshore wind power, which has constructed the wind farms on the sea, is less constrained than onshore wind power generation, making it possible to construct large–scale wind farms. Accordingly, interest in offshore wind power has steadily increased. In addition, the high electricity production cost compared to that of onshore, which has been pointed out as a disadvantage of offshore wind power, is gradually decreasing the gap, and its growth is evident with the development of high-capacity wind power technology. The Offshore wind power has been developed in European countries such as Germany, the United Kingdom and Denmark, which have abundant experience and technology with offshore wind power. The offshore wind power began in Denmark in 1991 with the construction of 11 offshore wind generators, the world’s first 40m high. Since then, it has been developed around countries close to the oceans such as the UK, Germany and the Netherlands with an annual increase of 30 percent over the past five years, with 23 GW of facilities as of 6 2018. Recently, the U.S. and China have also entered the offshore wind industry market and are expected to play a role as the next major countries.

Source : BloombergNEF H2 2018 LCOE Update - Wind

Figure 2.3: Levelized Cost of Electricity(LCOE) of Onshore and Oﬀshore

19 +31%

14 12

+26% 8 7 5 4

Figure 2.4: Capacity of Offshore Wind Power [2] 7 2.2 The Configuration of Offshore wind power systems

The typical layout of an oﬀshore wind farm is depicted in Fig.2.5. An oﬀshore wind power system is largely composed of 4 parts. 1st part is wind farm consisting of hundreds of wind turbines, 2nd part is wind farm collector system which is inter-turbine medium voltage ac cables (typically 33kV) , substation platform with transformer and converter platform for HVDC, 3rd part is transmission to shore like HVDC submarine cable and onshore converter station for HVDC, and last part is onshore AC main grid system.

... Offshore AC Grid ... 33 kV/100~230kV ...

... Transformer Platform1 HVDC ... Cable Onshore 33 kV/100~230kV ... VSC-HVDC HVDC VSC-HVDC AC Main Grid Offshore Onshore ...

... Transformer Platform2

Wind farm Wind farm collector system Transmission to shore Onshore grid

Figure 2.5: The typical conﬁguration diagram of oﬀshore wind farm

2.2.1 The reason for applying HVDC system to oﬀshore wind farms

In offshore wind systems, submarine cables are applied. When the AC submarine cable is transmitted in the long line, the amount of power transmission decreases due to the loss of reactive power and cable skin current due to the charging current. There- fore, reactive power compensation such as shunt reactors for long distance AC cables are required to maintain an acceptable steady-state voltage for the cable. If the cable is too long and there is not enough compensation at each end, the compensation will face technical and economic challenges. For this reason, a break-even distance is reached at approximately 20-40 km between HVAC and HVDC [13]. Fig.2.6 shows the configuration of the application of HVDC and HVAC according to the distance of the 8 offshore wind farm from the land. HVDC also has technical advantages over HVAC. The HVDC-link limits the fault spread from the wind farm to the onshore main grid, and vice versa. Therefore, HVDC transmission can be an excellent solution for offshore wind power generation systems.[14]

shore Offshore platform HVAC

< 20km

Wind Farm1 Offshore AC Main Grid platform HVDC

>20km Rectifier Inverter

Wind Farm2

Figure 2.6: The conﬁguration of HVDC connection

2.3 Wind turbines

A wind turbine is a device that produces electrical energy using wind energy. It consists of a turbine rotor, gearbox, generator, power converter and transformer for grid connection. The blades of the wind turbine convert the kinetic energy of the wind into mechanical energy, which is then converted into electric energy by the generator and finally passed through a transformer or power converter to suit the linkage of the power system. These relate to controls that meet the regulations for grid code requirements. Wind turbines can be divided into four types: fixed-speed wind turbines (Type-1), Lim- ited speed wind turbines (Type-2), variable speed with partial-scale turbines (Type-3) 9 and variable speed with full-scale turbines (Type-4) [15]. Since offshore wind farms are connected by large-capacity HVDC submarine cables, in most cases Type-3 or Type-4 generators are applied, but recently, wind turbines have been applied to Full Converter Type-4 synchronous generators.

2.3.1 Type-1 : The ﬁxed speed turbine systems

A typical configuration of a fixed speed wind turbine is shown in Fig.2.7. Wind turbines in this system operate at low speeds and are connected to the grid via transformers using squirrel cage induction generators (SCIG). The capacitor bank is used to compensate for the reactive power consumed by the generator. It is considerably economical and stable in terms of cost and has a relatively simple structure. However,compared with the variable speed wind power generation system, maximum efficiency can be achieved only at a given wind speed, so the wind energy conversion efficiency is low, and reactive power control is difficult. In addition, since the SCIG wind turbine is directly connected to the grid, it is difficult to meet grid connection regulations in the event of a grid fault [16]. Therefore, it is not suitable for offshore wind turbines.

SCIG Transformer Grid

Gearbox

Wind turbine Capacitor Bank

Figure 2.7: Type1-SCIG a ﬁxed-speed wind turbine [15]

2.3.2 Type-2 : The limited variable speed turbine systems

The conﬁguration of the limited variable speed turbine system with external variable resistance is shown in Fig.2.8. Similar to a Type-1 wind turbine, this system employs a wound rotor induction generator (WRIG) and can operate at various speeds by adjusting the rotor resistance. Torque and slip are controlled by adjusting the rotor resistance, and 10 it can produce constant output at a higher wind speed than rated wind speed. However, the range is limited to within 10% as heat loss may occur due to rotor resistance as the wind speed increases. The main advantage of this system lies in its simple structure and economics. However, it is diﬃcult to meet the LVRT requirements because the drive speed is limited and the reactive power on the system side is not controlled. [15].

WRIG Transformer Grid

Gearbox

Wind R turbine Capacitor Bank Variable Resistor

Figure 2.8: Type2-WRIG variable speed wind turbine [15]

2.3.3 Type-3 : variable speed with partial-scale turbine

This type is called the doubly fed induction generator(DFIG) and its configuration is shown in Fig.2.9. The stator of the wind turbine is connected to the primary winding of the transformer, and the rotor is connected to the third winding of the transformer via a power converter. Converters connected to the rotor are commonly used with two-level VSC back-to-back converters and the capacity is determined at 25-30% of the generator’s rating [17]. It controls the torque or rotational speed of the generator by using the amount of power generated by the converter connected to the rotor side, and is responsible for controlling reactive power flowing from the stator by supplying excitation current to the generator [18]. The advantage of this system is that it can operate at all wind speeds and has high power conversion efficiency. However, a gearbox is required due to the difference between the speed of the dual excitation induction generator and the speed of the wind turbine. The operation of the gearbox causes disadvantages such as loss of friction heat, cost of maintenance, noise, etc. In addition, the control method to meet the LVRT requirements in case of a grid fault is considerably complex [17]. 11

WRIG Transformer Grid

Gearbox

VSC VSC Wind R turbine + C Crowbar

Figure 2.9: Type3- DFIG wind turbine [15]

2.3.4 Type-4 : variable speed with full-scale turbine

Fig.2.10 shows the configuration of a full-scale wind turbine system. Type-4 generators are divided into conventional synchronous generators using electromagnets and permanent magnet synchronous generators (PMSG) that do not require excitation. Most of the permanent synchronous generators are applied to large capacity offshore wind farms. The PMSG is connected to the power converter via a stator winding, allowing independent active and reactive power control. It is referred to as a full-scale converter because all power flows through the converter as shown in the Fig.2.7. Due to the full-scale power conversion devices, the loss of power is greater and the cost is higher than that of DFIG. However, since the flow of power is controlled, the frequency of the generator can change without changing the frequency of the system. It also has advantages in controlling active/reactive power and complying with grid codes [17]. Recently, it has attracted attention as a wind turbine for large-scale offshore wind farms due to the price competitiveness and technological development of power converters.

2.3.5 Wind turbine control system

Wind turbine control employs power electronics devices to control the power flow. Wind turbine control methods for large offshore wind farms have various considerations, but they are generally determined by considering economic efficiency, power loss and grid code. Fully-rated converters are considered suitable to fulfill current grid code requirements [19]. Fig.2.11 shows a diagram of a wind farm connected to the VSC-HVDC link. The wind farm consists of a PMSG type fully-rated IGBT based 2-level back-to-back 12

VSC VSC PMSG Transformer Grid + C

Wind turbine

Figure 2.10: Type4-PMSG a fully rated converter-connected wind turbine [15] converter. The VSC-HVDC link is connected by DC cables between two converter stations: offshore and onshore converter station. The power produced by the wind farm is collected at the offshore converter station, and the HVDC-link delivers the power produced by the wind farm to the onshore main grid via the HVDC cable and onshore converter station. In large wind farms, HVDC-link power converters independently control reactive power in wind farms and onshore grids. This controls the bulk active power and delivers it to the grid [19]. Fig.2.12 shows the overall control diagram of the offshore wind farm. The wind turbine control can perform in two control modes: power control and voltage control. An offshore VSC-Converter converts AC power output from wind power generator to DC and transfers it to HVDC-link converter. The grid-side converter receives DC power, converts it to AC power, and transmits it to the grid. The power control method aims to deliver the power to the grid by tracking the maximum output of wind energy. In the voltage control mode, the DC voltage is controlled and the output voltage is limited [18]. 13

VSC back-to-back converter O¡ shore Onshore converter converter PWF P

G HVDC Cable VSC back-to-back converter Q Main Grid QWF HVDC

VSC back-to-back converter

Figure 2.11: Wind turbines connected to grid via HVDC

2 level-VSC O¡shore HVDC back-to-back Converter MMC1 PMSG

VDC -

va vb vc ia ib ic va vb vc ia ib ic PLL PWM PWM PLL θ θ

Pitch abc abc abc abc Varef Vbref Vcref Varef Vbref Vcref controller dq dq dq dq abc abc dq dq

Wind Power vd vq i d i q vd vq i d i q controller vdref vqref vdref vqref

Pref i qref i qref Pref Power Current Current Power Controller Controller Controller i dref Controller Qref i dref Qref

VDC VDC DC Voltage VDC_ref Controller

Figure 2.12: Control diagram of oﬀshore wind farms 14 2.3.6 Oﬀshore AC grid

An offshore AC grid can be divided into an internal grid and offshore substation. The internal grid consists of a number of wind turbines and submarine cables that connect them to collect power generated by wind turbines. These cables are connected to the transformers in the wind turbines. The power produced by wind turbines is transmitted to the offshore substation via cables. The voltage level of the internal power grid applies nominal voltage level of 30-36kV in most cases. Most wind turbines produce power at voltage levels below 1000 V and are boosted to internal grid voltage levels by transformers installed under the turbine. The offshore wind farms far from the coast can carry out efficient power transmission to the onshore main grid system by raising the voltage of the internal power grid from the offshore substation to reduce the cost and loss of power cables.

2.3.7 VSC-HVDC converters : oﬀshore and onshore station

Installation of offshore and onshore converter stations is necessary for long-distance HVDC transmission, which is at least several tens of kilometers or even hundreds of kilometers. Fig.2.13 illustrates the control structure for point to point HVDC connecting an offshore wind farm [14]. The offshore converter station is installed in the ocean, and takes the role of collecting the power produced by the wind farm and converting it from AC to DC. The offshore converter station controls the voltage and frequency of the offshore AC grid. The onshore converter station is located on the mainland and converts DC current to AC and transmits it to the onshore AC main grid. For this purpose, the onshore station must control the DC link voltage [14]. The HVDC-link must deliver all the power produced at the wind farm, which requires a control system to deliver the power. More details regarding the HVDC system will be covered in the following chapter. 15 [Onshore Station] [O shore Station] Maintain Constant AC Voltage Control DC Voltage Terminal 1 DC Cable Terminal 2 AC DC

Grid VDC DC AC Offshore IGrid Wind Farms

VGrid Power PLL DC Voltage MMC PWM Measurement MMC PWM

Converter1 Converter2 current control current control

I_conv_ref I_conv_ref DC Voltage Control AC Voltage Control

VDC VDC_ref Pref Qref Vref

Figure 2.13: Control structure for point to point HVDC connecting an oﬀshore wind farm [14]

2.3.8 HVDC Cable

The VSC-HVDC cable is applied with a cross-linked polyethylene (XLPE) cable. XLPE cable has the advantage of low cost and excellent workability. However, LCC-based HVDC can cause cable damage due to the current polarity reversal problem of the converter, so it is inevitable to apply Mass-Impregnated (MI) cables that are expensive and disadvantageous for construction [20]. This is also one of the advantages of applying VSC-HVDC. Chapter 3

HVDC system for oﬀshore wind farms

High Voltage Direct Current (HVDC) transmission is a technology that enables direct current transmission by converting alternating current into direct current in a converter consisting of a power-electronic switch and then by converting direct current back to alternating current. HVDC has been used for more than 50 years in interconnecting asynchronous networks, transporting long distance energy, and submarine cables. Along with the rapid interest and increasing demand for renewable energy generation around the world, there has been growing interest in the HVDC system, especially in the oﬀshore wind power ﬁeld where large capacity long distance transmission is required.

3.1 Voltage sourced converter for HVDC

Depending on the type of power-electronic switch used for conversion, two types of HVDC transmission systems are used today: voltage source converter (VSC) using in- sulated gate bipolar transistors (IGBT) and line-commutated converter (LCC) HVDC [21]. The LCC-HVDC type has the advantage of low conversion loss, which is advantageous for large capacity and low price, but has the disadvantage of requiring reactive power compensation. Although the VSC-HVDC type has a disadvantage in that the conversion loss is larger than that of the LCC type, there is no need to supply reactive power, and the active power and reactive power can be controlled independently.

16 17 Therefore, the VSC-HVDC system has the advantage of satisfying the low voltage ride through (LVRT) regulation when it is connected with the oﬀshore wind farm [21]. This chapter focuses on VSC-HVDC, which is widely applied in oﬀshore wind farms. At present, major manufacturers of HVDC can apply up to 2.0GW ±500kV using the VSC method and plan to expand up to a ±515kV level within a few years [3].

3.2 VSC converter conﬁgurations

Three types of converter conﬁgurations are typically used for VSC-HVDC systems: 2- level converter, 3-level converter and and modular multi-level converter (MMC) [15].

3.2.1 2–Level converter

The 2-level VSC-HVDC, the basic method of VSC-HVDC, consists of a three-phase half-bridge converter as shown in Fig.3.1. Since the AC voltage of each phase has a voltage equal to the positive and negative voltage of the DC voltage, it is called a 2- level converter. If the upper valve of one phase is turned on, the AC output terminal is connected to the positive DC terminal and becomes +Vd/2 to the zero point of the converter. However, if the bottom valve of one phase is turned on, the AC output terminal is connected to the negative DC terminal and becomes -Vd/2 to the zero point of the converter. The output waveform produced by the two-level converter is a quadra- ture waveform, containing harmonics above the allowable values. In order to improve harmonic distortion, it is necessary to switch the IGBT in PWM mode during one cycle of the alternating current. In this case, high switching losses occur in the IGBT, which reduces the overall transmission eﬃciency. There are several types of PWM applicable to HVDC systems, but all are less eﬃcient than thyristor converters due to their high switching losses. In addition, in order to apply to HVDC, the circuit design is quite complicated because the operating voltage is high, so hundreds of IGBTs per valve must be connected in a series and be switched simultaneously.

3.2.2 3–Level converter

To improve the harmonic performance and rapid voltage ﬂuctuations of 2-level converters, 3-level converters are used in some VSC-HVDC as shown in Fig.3.2. The most 18

VDC

L1 UL1

neutral L2 UL2

L3 UL3

VDC

Figure 3.1: Scheme of a 2-level VSC common three-level converters are diode clamps or neutral clamps, each phase consisting of four IGBT valves and two clamp diodes. The DC capacitor consists of two clamped diodes connected in a series by connecting the midpoint and 1/4 and 3/4 of the IGBT valve on each phase. Two switches of the upper arm are turned on to obtain a positive output voltage (+Vd/2), and two switches of the lower arm are turned on to obtain a negative output voltage (-Vd/2). Then, they turn on the two IGBT valves located inside to obtain a zero output voltage and connect them to the midpoint of the capacitor through a clamp diode.

3.2.3 Modular Multi-level Converter (MMC)

Compared to 2-level or 3-level converters, MMC has excellent output wave-forms near sine waves, high efficiency due to the low switching frequency of IGBT and a high capacity converter configuration due to its modularity. Each valve of the two-level converter consists of a large number of series-connected IGBTs and is a single control switch operating at high voltages, whereas each valve in the MMC can achieve the desired voltage by adjusting the number of sub-modules that are inserted. It is recognized as the most suitable topology and considerable research has been done on the topology configuration and operation techniques of the relevant MMC [21]. 19 +

L1 UL1 L2 UL2

Neutral L3 UL3 Point

Vd - Figure 3.2: Scheme of a 3-level VSC

3.2.3.1 Operation principle of MMC

The MMC method is a structure in which a Sub-Module (SM) composed of a half bridge converter or a full bridge converter is connected in a series. Fig.3.3 shows a typical configuration of the three-phase MMC. An independent phase unit is connected in three, and one phase consists of an upper arm and a lower arm. Each arm consists of a large number of independent sub-modules, each of which has an energy storage capacitor. Each sub-module has the most common half-bridge configuration, in which two IGBTs are connected in series and capacitors are connected at both ends. The operation of the sub-modules is the same as for a two-level converter, where each sub- module generates zero or capacitor voltage. Thus, if the number of sub-modules per valve is large enough, the valve will form a stepped waveform quite close to the sine wave, as shown in Fig.3.4. The leg configuration of MMC is a structure in which unit modules are connected in series in proportion to the size of output voltage and the number of levels. The voltage of each arm is formed by the sum of the output voltages of each sub-module, and the output voltages of the upper arm and the lower arm are combined to form the output voltage of the AC terminal. If balancing control of all sub-module capacitor voltages is performed, the capacitor voltage of each sub-module is uniformly formed by dividing the voltage of the entire DC terminal by the number of sub-modules of one arm. In MMC, the maximum number of levels of AC output voltage is related to the number 20

SM SM SM SM + 1 1 1 S1 D1 SM SM SM 2 2 2

S D 2 2 — SM SM SM n n n Sub Module Upper Arm

Arm inductor

Vterminal

Phase Unit

SM SM SM 1 1 1

SM SM SM 2 2 2 Lower Arm SM SM SM n n n Leg

Figure 3.3: Structure of MMC HVDC of sub-modules per arm. If there are N sub-modules for each arm, the number of levels of output voltages becomes N + 1. Each level is generated according to the number of sub-modules in the upper and lower arm, and N sub-modules can be on-state or oﬀ-state. This ensures that the DC-link voltage always has a constant value. As shown in Fig.3.5, the value of each level is Vdc/N [23]. 21

5Vdc

2π π /2 0 0 3π /2

− 5Vdc

V dc P5 0 − P5 Vdc P4 P4

P3 P3

P2 P2

P1 P1

Figure 3.4: Output waveform generated from 11-level converter with 5 DC sources [22]

Level 1 + Level 2

Vref

Vdc N 0 T

- Level N+1

Figure 3.5: Output waveform of MMC 22 3.3 MMC-HVDC controls

In general, in the VSC-HVDC system, the converter stations at both ends control the active power by changing the phase between the AC voltage of the converter and the

filter. One station controls DC-link voltage Vdc and the other station controls active power Pdc in the HVDC system. For the case of the VSC-HVDC system, the lead or lag control of the reactive power (Q) is also possible through the firing angle control [24]. Fig.3.6 depicts the entire control block diagram of the MMC-HVDC. The control structure of a typical VSC-HVDC system consists of a fast internal current control loop and an outer control loop, and the active and reactive power are adjusted by the phase angle and amplitude of the converter current with respect to the Point of Common Coupling (PCC) voltage. MMC1 receives reference values of active and reactive power, and MMC2 operates by obtaining reference values of DC voltage and reactive power. The outer control regulates the power transmission between the AC and DC systems and calculates the current reference value so as to follow the reference value of the block and inputs it to the inner control block. Inner control includes a current controller and processes the reference values. It is converted to the voltage value that must be output from the MMC-HVDC. Finally, the voltage reference value determines when the SM connected to the arm is turned on or off through the voltage balance algorithm of the SPWM and the SM.

3.3.1 Current controller

The inner current controller is a reference tracking method that separates active and reactive power by controlling the current component Idq independently [25]. In the current controller, the current ﬂowing through the terminal is converted into the d-axis and q-axis. It is controlled by the d-axis reference current value from the DC voltage and active power controller, and the q-axis reference current value from the AC voltage and reactive power controller. The current controller of the VSC reported in the literature can be represented by the diagram illustrated in Fig.3.7. A design of the inner current 23 AC Grid1 MMC1 MMC2 AC Grid2

VDC -

va vb vc ia ib ic va vb vc ia ib ic SM Voltage SM Voltage PLL PLL Balance Balance θ θ

abc abc SPWM DC Voltage SPWM abc abc dq dq Measure dq dq

vd vq i d i q Varef Vbref Vcref Varef Vbref Vcref vd vq i d i q abc abc dq dq

i qref vdref vdref i qref Pref Pref Power Current Current Power Controller i dref Controller vqref vqref Controller i dref Controller Qref Qref (Outer Control) (Inner Control) (Inner Control) (Outer Control)

VDC VDC DC Voltage VDC_ref Controller (Outer Control)

Figure 3.6: Control block diagram of MMC-HVDC controller loop can be made from the following equation [26].

d L id = iqωL − idR + Vtd − Vsd dt (3.1) d L i = i ωL − i R + V − V dt q d q tq sq where id and iq are state variables (output current), L and R are the impedance of the system, Vt is the terminal voltage (control input), and Vs is grid voltage (disturbance input).

3.3.2 DC-link voltage control of MMC-HVDC

The DC-link voltage needs to be controlled for stable operation of the MMC-HVDC.

Fig.3.8 shows the equivalent circuit of DC-link. If converter’s outflow power is Pin, capacitor’s inflow power is Pc, grid’s inflow power is Pout, Power Flow Dynamic is as shown in equation (3.2). When a difference between the active power input and output occurs, a change appears in the DC voltage. The main function of the DC-link voltage 24 vd

id*

iq*

Figure 3.7: Control block diagram of a current-controller controller is to keep the DC-link voltage constant. The DC-link voltage set point is typically ﬁxed at 1.0[p.u]. The DC voltage is set by the d-axis current control, and the control output is set to the d-axis current reference value by selecting the error between the DC voltage reference value and the actual DC voltage as the control input. The set d-axis current reference value is input to the MMC master controller to control the required active power. As a result, the DC voltage is controlled.

PIn POut

Ceq Vdc

VSC-HVDC VSC-HVDC RECTIFIER INVERTER Figure 3.8: Electric equivalent circuit of DC-link

1 dV 2 P = P − P = C dc (3.2) C in out 2 dc dt 25 3.3.3 Concept of HVDC system controls connected to wind farms

The HVDC connected to the wind farm performs simultaneous maximum output control based on the wind power-followed control. HVDC converter station will basically control the maximum output of the wind farm considering the wind speed in steady state and perform fault ride-through (FRT) operation that maintains possible output operation in consideration of system stability in case of a fault in a nearby AC system [14]. When the FRT is exceeded, the wind farm will shut down. The generator output target for maximum power point tracking (MPPT) is determined based on the generator speed and the output (P-W) curve. Overall, this means that cooperative control of the wind turbine and the HVDC-Link is carried out. In the HVDC converter station connected to the wind farm, the wind farm side converter station controls the AC voltage and Pdc, and the main grid side converter station controls the AC and DC voltage, respectively. This is to reﬂect the maximum output control of the wind farm based on the wind power-followed control. The wind farm side converter controls the magnitude and phase of the AC bus voltage at the station, while the grid side converter controls the DC voltage within the rated voltage range through PWM phase control of the AC voltage. In addition, the reactive power is controlled through PWM control of the AC voltage [15]. The wind farm side converter controls DC transmission power corresponding to wind power through PWM phase control of AC voltage. To control the DC transmission power means controlling the DC voltage to the target value at the wind farm side that generates the DC current in consideration of the DC voltage and the line resistance. Except for the following conditions, power of wind farms produced at that time is transmitted to the grid side at all times. If the grid frequency rises above a certain level and there is a need to reduce the output of the wind farm, the power generation is reduced through the pitch control and the transmission power is also reduced. If the voltage decreases below the FRT requirement due to a fault in the wind farm or the associated equipment, wind power generation is stopped. If the DC voltage of the wind farm side converter increases excessively due to a fault on the grid side, and it is necessary to protect the DC capacitor from overheating and to reduce wind power generation. 26 3.3.3.1 Onshore Grid Side Converter

The main function of an onshore converter associated with an offshore wind farm is to control the DC-link voltage [14]. It performs the function by measuring the voltage, current and frequency on the main land grid side and DC voltage of HVDC-link. The HVDC-link voltage control is an important factor in determining the power balance and Fault Ride-Through capability [24]. Hence, a proper design must be made and it is important to define the operating range not to exceed the operating limit within the reference value of d, q-axis current. If the DC-link voltage is not controlled at an appropriate level, the active power mode cannot be maintained. Fig.3.9 depicts the control conceptual diagram of the onshore grid side converter. Where Eac is the main ∗ grid AC voltage, Eac is the reference value of AC main grid, Edc is the DC-link voltage ∗ value, and Edc is the reference voltage of DC-link. The difference between the measured DC voltage and the reference DC voltage is used as the input of the PI controller to make the d-axis current reference value. The AC voltage and AC voltage reference values measured at the grid side PCC are used as inputs to the PI controller, creating a q-axis current reference value. These values are entered as a new d, q-axis voltage reference value through the current controller and a new AC three-phase reference value is generated through the inverse d, q conversion [26].

3.3.3.2 Oﬀshore Grid Side Converter

In a typical point-to-point HVDC, one station controls the power and the other station controls the DC voltage. In offshore systems, however, offshore grid side converters control offshore grid voltage amplitude and frequency [14]. The offshore converter is operated and controlled as a voltage source with a constant phase angle and AC frequency. [27]. The converter connected to the offshore wind power measures the voltage, current and frequency of the PCC so that the reference value of the d-axis current is 0 and the AC voltage is stabilized by the q-axis current [26]. Fig3.10 depicts the control diagram of the offshore grid side converter. 27

Onshore Main Grid MMC-VSC

Vdc iabc ,Vabc

abc dq PWM θ idq,Vdq Current - Vdc PI Controller

- Vdc* PI Vac

Vac* Figure 3.9: The control conceptual diagram of Onshore grid side converter

Offshore Windfarm MMC-VSC grid PCC

Vac

θ ref PWM

Vac - Iq* Current PI Controller

Vac* Id Figure 3.10: The control conceptual diagram of Oﬀshore grid side converter Chapter 4

Low Voltage Ride-Through (LVRT)

The Grid-Code of wind farms can be defined as connection guidelines for specific power systems stipulated to connect to the AC system while ensuring a certain level of electrical quality and system stability in regard to the offshore wind farms with power vulnerability [4]. A characteristic of wind power generation is that it is onerous to change the output power and is basically un-dispatchable since the output depends on the wind speed even if it is partially supplemented by the battery. Although the system connection method of large offshore wind farms takes into account a variety of factors, it is usually determined in consideration of the grid code by reflecting the economic feasibility, power loss, and the characteristics of the AC power system connected.

4.1 The Overview of grid codes

The specific application value of Grid-Code is different for each country and varies depending on the system conditions, but the basic contents and concepts contained in Grid-Code are similar. The following is the general definition of the grid code. Depend- ing on the situation in each country, all or part of the following technical standards are specified in the grid code. An overview of the important technical items that make up the grid code is shown below.

28 29 • Fault Ride-Through during a Grid Fault

• Active Power / Frequency Control standard (normal / fault)

• Reactive power / voltage maintenance standard (normal / fault)

• Electrical quality standards (ﬂicker, harmonics, voltage unbalance standards, etc.)

• Communication / information exchange standard

• Other protective cooperation regulations, etc.

Due to the large-scale wind farms, stability and power quality must be taken into consideration when connecting to the grid. In countries around the world, grid codes have been applied when connecting oﬀshore wind farms with intermittent generation characteristics to the grid. Among them, there is a low voltage ride-through (LVRT) requirement, which is the standard of operation of wind farms connected with a grid in case of a grid fault. In the case of an existing grid fault, the wind farm system uses a method of separating from the grid. However, due to the importance of new and renewable energy, the factors aﬀecting the system have increased because of the increment of capability of wind farms, the LVRT requirements were created to contribute to the recovery of grid faults. LVRT requirements are applied to determine the operation of the connection and disconnection of wind farms based on the rate of voltage drop and the fault duration in the event of a grid fault.

4.2 LVRT requirements during grid faults

Fault ride-through (FRT) is the mandatory requirement to operate a wind generator even if a fault occurs in a connected system above a constant voltage-time profile to ensure stability of the system operation. FRT is largely divided into LVRT that defines operation in a system low voltage state and high voltage ride-through (HVRT) that specifies over-voltage conditions, which are generally defined by the LVRT requirements. The purpose of specifying LVRT is to maintain system stability by continuing operation even if the wind generator is burdened at a voltage level above a certain level and a fault occurs in the grid. There are three parameters that define the FRT, the lowest 30 voltage in the event of a fault, the duration of the fault, and the voltage recovery time after clearing the fault as shown in Fig.4.1.

100 ms 1.2 1.1 1

No trip Voltage recovery after fault clearing during fault

V (pu) V Wind Acceptable Voltage Dip Acceptable Voltage generator may trip

0.150 0.625 1.5 1800 (30 min) Time(s) Fault Duration

Figure 4.1: Main parameters of FRT requirement

The voltage value that is the standard for determining the FRT is the point of common coupling (PCC) of the wind farm, which is instantaneously disconnected without applying the FRT at the time of internal fault of the wind farm. This means that the FRT is applied only in the event of an external fault and continuous operation should be possible when the value is higher than the reference value. According to the grid- code requirements of the world’s major countries, the lowest voltage standard for each country is deﬁned as 0-0.25(pu), 140-625(ms), and the recovery time of the voltage is 0.3-3.0(s). Each parameter is set up after a thorough system review, considering the conditions of the country’s system and the time of protection cooperation (previous time). Fig.4.2 illustrates the LVRT requirements of various grid codes of the major nations of the world [4]. The German requirement is the most stringent regulation that guarantees grid connection even if the voltage drops to zero and is considered a criterion for connection requirements in many countries[28]. Table.4.1 illustrates the characteristics for wind turbines in various grid codes. 31

Figure 4.2: LVRT requirements of various grid codes

Table 4.1: Fault ride-through capability for wind turbines in various grid codes

Requirement of FRT Grid code Fault duration Voltage drop level Recovery time

USA 625ms 15% 3.0s Canada(AESO) 625ms 15% 3.0s Germany(Eon) 150ms 0% 1.5s UK 140ms 0% 1.2s Spain 500ms 20% 1.0s Denmark(<100kV) 140ms 25% 0.75s Chapter 5

HVDC-link over-voltage control strategies

This chapter reviews the causes of over-voltage occurring in HVDC-link and the control strategies suggested in the existing research to solve these problems, and proposes new control strategies.

5.1 The reason for over-voltage of the HVDC-link

Fig.5.1 shows the over-voltage process of the HVDC-link. In general, in the event of a fault on the main grid connected to a large wind farm, the grid system will be temporarily unloaded. However, most grid codes require the wind generator to remain connected during a grid fault. Therefore, when a fault occurs in the onshore AC main grid, the AC grid voltage decreases, reducing the power transfer capability of the onshore grid. Since the power of wind generators cannot be reduced immediately, additional power is accumulated in the HVDC-link [29]. This causes the DC voltage to rise due to the accumulation of energy on the HVDC-link. The DC voltage range of HVDC systems is maintained within 5-10% in most utilities [30]. Hence, proper control strategies are needed to operate a stable HVDC system.

• Power transfer capability In the steady state, power of the capacitor is almost zero because most of the

32 33 generated power is transmitted to the grid. Therefore, DC voltage remains constant without large ﬂuctuations. The expression of the active power ﬂow can be expressed by Eq.5.1.[31] V ∗ V P = P − P = c g sin(δ − δ ) (5.1) C Gen Grid X c g Where,

PC : Active power of capacitor VC ∠δc : Onshore converter terminal voltage Vg∠δg : Main Grid terminal voltage X : Reactance

• DC Over–Voltages during AC faults If a fault occurs on the AC main grid side, active power is not supplied to the grid side as shown in the right figure of Fig.5.1. The power current flowing through the capacitor can be expressed as the difference between the wind power generation and the grid-side supply power. That is, the grid-side supply power decreases immediately while the wind-side power decreases linearly with a time lag. The

power is accumulated in the DC capacitor, increasing PC , as shown in Eq.5.2 [31].

1 dV 2 P − P = C dc Gen Grid 2 dt (5.2) s 2 Z VDC = (PGen − PGrid)dt Ceq Where,

Ceq : the equivalent capacitance of the DC line and the capacitor

5.2 The Existing over-voltage control methods

5.2.1 The application of a DC chopper

In this method, a DC chopper is applied to dissipate the accumulated energy, so the DC-link voltage can be stabilized quickly [10]. Braking resistors are connected in a 34

Figure 5.1: The power ﬂow at the DC-link dynamics during a AC main grid fault series as shown in Fig.5.2 and Fig.5.3 to suppress the DC-link voltage rise in case of a grid system fault [10]. In case of a fault on the grid system, DC voltage rises due to an imbalance between the input and output of the HVDC-link to satisfy LVRT requirements. The chopper resistor installed on the DC-link side determines whether the resistor is short-circuited and opened according to the DC voltage of the HVDC-link. Chopper resistors are used to dissipate excess power to a rise in the DC voltage within acceptable levels. The grid fault causes the voltage across the DC capacitor to increase to a higher value and the switch turns on via trigger pulse T. The voltage across the

DC capacitor is discharged through the resistor Rc. If the DC voltage rises above a certain level, switch T turns oﬀ. Thus, by using a DC chopper, the DC line voltage can be normalized in a controlled manner [32]. The main advantage of this method is that quick action is possible to keep the DC voltage stable [7]. However, heat is generated by using resistance to dissipate energy. To address these problems, operation should be minimized due to increased costs associated with the installation of cooling devices. In addition, it is necessary to solve the problem that can cause stress to the system instantaneously or large ﬂuctuations due to sudden surge.

T D

Figure 5.2: Basic diagram of DC Chopper 35 DC+(Cable) Terminal1 I Vn Rchopper Ceq Zn AC Main Grid Fault Onshore Chopper DC-(Cable) MMC-VSC Resistor Figure 5.3: Block diagram of control scheme

5.2.2 The wind power output control based on communication lines

This is a method to mitigate the over-voltage by reducing the electric power flowing from the wind power generator due to a non-transferable situation on the main grid. Reducing the electrical output of wind power can help reduce the imbalance of input and output power of the HVDC-link by reducing the amount of power flowing into the HVDC-link. Fig.5.4 shows the layout of wind power output control of an offshore wind farm in case of a main grid fault. When a fault occurs in the main grid, the command signal is transmitted to control the active power by adjusting the frequency, voltage, or reference power of the wind farm grid as shown in Fig.5.5. It relies on the communication line installed between the HVDC converter stations of the offshore wind farm to transmit control signals [7]. However, for this, the long-distance DC-link high-reliability communication line must be maintained, and there is a possibility that control may be delayed due to a potential communication delay problem [12]. There is also a need for a way to minimize the mechanical and electrical stress on the wind turbine [6]. 36 Wind Power Control

WTCU

G Communication Line SEC REC Controller Controller

WTCU

G VDC Main Grid HVDC Link

O¡ shore Onshore WTCU converter converter

Figure 5.4: The diagram of the oﬀshore wind power output control

Detect Fault

VDC >VTh

Yes

Wind Power Control

Power Reference Droop Control

Figure 5.5: The ﬂowchart of the wind power output control 37 5.3 The proposed control method

The proposed control strategy focuses on fast and reliable detecting of faults, minimizing DC Chopper application, and ﬁnding ways to improve over-voltage control performance while minimizing electrical and mechanical stress on wind power generators. The proposed control schemes are added as shown in the gray area of Fig.5.6 in the existing control scheme.

Maintain Constant AC Voltage Control DC Voltage DC Cable AC DC Grid VDC DC AC Oﬀshore Wind Farms

MMC SPWM MMC SPWM Power PLL

Converter1 Converter2 current control current control

I_conv_ref I_conv_ref DC Voltage Control AC Voltage Control

VDC VDC_ref Pref Qref Vref

Figure 5.6: The control scheme of proposed method

5.3.1 Fast fault detection

The existing fault detection schemes described in the literature [11][21][33] during a main grid fault are based on the voltage rise of HVDC-link through a communication line connected to onshore and offshore converters. The dynamics of long cable capacitors interfere with keeping the DC voltage at a low level, and fault detection occurs when 38 it is above acceptable levels. Furthermore, the DC voltage should be controlled over a permissible level to prevent malfunctions in fault detection[34]. Since the capacitive current in cables can cause circuit breakers to malfunction during voltage transients, the application of current differential protection, which is widely used in AC systems, is also problematic to employ to a DC link [35]. The proposed method hinges on the voltage fluctuation of the AC main grid and offshore grid during a fault. When a main grid fault occurs, the system voltage temporarily drops. As the power from the wind farm is not transmitted to the grid, the offshore grid AC voltage as well as the DC voltage of the HVDC-link increases. Fig.5.7 and Fig.5.8 show the simulation results of the AC main grid voltage and offshore grid voltage in the event of an AC main grid fault. In the proposed method, the voltage fluctuation of the AC main grid and offshore AC grid is observed, and the command signal is transmitted when it detects voltage above or below a predetermined value as shown in Fig.5.9. The offshore grid voltage detection method can catch the main grid fault in the wind farm inside, which can contribute to solving the risk of communication delays due to the unavailability of a communication system between onshore and offshore converters on long lines of more than 100 km.

Fault occurs

Figure 5.7: AC main grid voltage during a fault 39

Fault occurs

Figure 5.8: Oﬀshore grid AC voltage during a fault

Detect Fault

ADD

VGrid < VMin VWind>VMax VDC >VTh

Control Signal Transmission Figure 5.9: New fault detection algorithm

5.3.2 DC-link current control in the main controller

The proposed method is based on the fast stabilization of the desired DC voltage by adding a d-axis current limiting function to the HVDC controller. The gray areas in Fig.5.10 show the overall proposed control scheme added in the existing control system. In an oﬀshore VSC-HVDC system, one station controls the DC-link voltage and the other controls the active power or frequency [36]. The output voltage magnitude and angle of MMC-HVDC can be controlled indirectly using d-q current control. This is 40 because it can limit the valve current under balanced operating conditions and provide a faster response than direct control. In d-q control, the d-axis current control target is set to DC voltage, and the error between the target DC voltage command value and the actual DC voltage is selected as the control input. The desired control output is set to the d-axis reference current. The set d-axis current reference is input to the MMC controller and controls the required active power and DC voltage. Fig.5.12 shows the basic control structure for an MMC-HVDC. The d-q current controller achieves the d,q axis decoupling through the PI controller and can be expressed as Eq.5.3 [37].

∗ did Ki ∗ Vd = −(L + Rid) = (Kp + )(id − id) dt s (5.3) di K V ∗ = −(L q + Ri ) = (K + i )(i ∗ − i ) q dt q p s q q

AC Grid MMC-VSC 230/370kV

VDC - 230kV

va vb vc ia ib ic

Varef Vbref Vcref abc dq abc abc dq dq PI VDCref

Power Control Loop

Figure 5.10: The current control scheme in main controller

The transient analysis results show that the d-axis current ﬂuctuates severely during the main grid fault, as shown in Fig.5.13. Based on the analysis results, a d-axis current limiting function that allows the d-axis current to be adjusted within acceptable level 41 idmax - id PI vdref idref -idmax Figure 5.11: The structure of current limiting function

*/ * * * * P Vdc / f Outer idq dq current Vdq MMC V * * Q/ Vac Controller Controller Controller

Figure 5.12: Basic overview of MMC control system [36] is added. This function helps the DC-link voltage recover quickly by limiting excessive changes in the d-axis current.

Fault occurs

Figure 5.13: Analysis of d-axis current during 1-phase fault

5.3.3 Wind power control by current droop

Fig.5.14 shows the control strategy of the wind side converter. In the proposed method, the offshore wind farm detects the main grid fault by itself and controls the wind power through the current droop. An efficient solution to improve the LVRT capability of the VSC-HVDC is to minimize the power delivered to the HVDC link under voltage sags [21]. When a main grid fault occurs, the voltage at the offshore wind farm and the 42 AC grid of HVDC terminal point rises as described in the Fault Detection Section and the d-axis current between the wind turbine converters changes considerably as shown in Fig.5.15. The fault detector catches offshore grid voltages beyond the permissible range, and the converter controller calculates the amount of change in the d-axis current, rapidly reducing wind power through current droop control. Since the AC voltage at the offshore wind farm is always measured for the control of the wind turbine, it has the advantage that this control strategy is possible regardless of the communication system between the HVDC-link converter requiring long distance communication and an effective power reduction can be obtained.

Figure 5.14: Control scheme on wind farm side converter 43

Fault occurs

Figure 5.15: Analysis of d-axis current during 1-phase fault at the wind farm side

5.3.4 Cooperative control with existing method

This method is applied in coordination with the control method of the chopper resistor which is effective to suppress the peak value of voltage together with the method suggested in the previous section. If an extreme rise in DC voltage occurs, a reduction in active power may not be enough to mitigate it. Especially in the case of extreme fluctuations such as a 3-phase fault, as shown in Fig.6.6, the peak value of the voltage should be controlled quickly. Therefore, if the DC voltage exceeds 1.15 p.u, control is performed in cooperation with the DC chopper as shown in Fig.5.16. In this case, the advanced results can be achieved by cooperating with each of the advantages of the quick control method and the Chopper Resistor mentioned in the previous section. In addition, compared to using only chopper resistors, since the wind power de-loading control is applied concurrently, the amount of energy to be dissipated using the chopper resistor is reduced, so the current flowing through the resistors also decreases. Hence, the burden of heat dissipation is lowered. Thus, there is an advantage of reducing the installation cost such as a cooling device to control heat generation. 44

Detect Fault

VGrid < VMin VWind>VMax VDC >VTh Yes Yes Yes

No VDC>1.15p.u

Yes

Use of DC Chopper

VSC1 Control Wind Power Control

DC Line Deloading Current Limiting by Current Droop

Figure 5.16: Flowchart of the proposed control method Chapter 6

Simulation

6.1 Simulation study

The MMC-HVDC system connected to a large oﬀshore wind farm and its control method are simulated by PSCAD/EMTDC in this research [38]. The PSCAD/EMTDC program was created by the Manitoba Research Center in Canada and can simulate a power system or an HVDC facility. In addition, the fault of various AC power systems or HVDC conversion facilities can be simulated according to various causes. It is also used to identify the cause of the fault. PSCAD are also used as one of the simulation software tools in major manufacturers related to HVDC such as Siemens, ABB and GE. Fig.6.1 and Fig.6.2 depict the entire simulation system. In all simulation cases, the wind speed is set at a constant speed of 10m/s to ignore the eﬀects of the external elements except for a fault in the main grid.

DC 640kV Terminal2 AC Terminal1 230/33kV Main Grid 100km Y Y 230/370kV PMSG - 450MW 10km (Wind Farm 1) Y

230kV 230/33kV Onshore Oﬀshore Y Y 10km PMSG - 450MW MMC-VSC MMC-VSC (Wind Farm 2)

Figure 6.1: The physical simulation model

45 46

Onshore_Frequency(initial:50Hz)

Onshore Freque Terminal 2 P = -849.3 MMC VSC P = 878.5 Q = -63.1 Q = -217.1 P = 441 P = 446.4 AC Grid Terminal 1 V = 229.9 V = 227.3 Q = -54.22 475 [MVA] Q = 8.998 (Onshore) 230 kV(AC) V = 227.5 230 / 33 V = 32.84 A A COUPLED AC_Network Net Wind V V A #1 #2 A V V 100 km PI PMSG Systems (DC) 10 km SECTION (Wind Farm 1) 640kV ﬀ Onshore HVDC_link O shore

230 kV(AC) Vbus Converter Converter AC_flt_type AC_flt_type S i S i AC_flt_3 AC_flt_1 450MW

P = 441 P = 446.6 Q = -95.91 475 [MVA] Q = -31.38 V = 227.4 230 / 33 V = 32.47 COUPLED A #1 #2 A V PI V AC_flt_type PMSG Systems 10 km SECTION (Wind Farm 2) AC_flt_2 AC_flt_type

AC_ﬂt_4 450MW

Figure 6.2: The simulation model in PSCAD/EMTDC

Table 6.1 shows parameters of the simulated MMC-HVDC system. The system can be divided into 3 parts according to the main function : wind farms, MMC-HVDC and power grid.

Table 6.1: Parameters of MMC–HVDC System Model 900MW Power Rate (Wind Farm1 :450MW, Wind Farm2:450MW) Wind Farms 10km Interconnected grid (AC Cable 230kV) Transformer 230kV/33kV Wind Turbine Type Type-4 : PMSG Rated Capacity 950MW Cell Capacitor 2800uF MMC Number of Submodules 76/arm HVDC Rated DC Voltage 640kV DC link length 100km Rated Voltage AC 230kV Power Grid Nominal Frequency 50[Hz]

6.2 Validation of case studies

To verify the eﬀect of the proposed method, a MMC-HVDC system connected to oﬀshore wind farms is simulated in PSCAD/EMTDC. 47 6.2.1 Normal Operation

The simulation during normal operation is performed to verify if the simulation model is performed properly. As for the AC grid and HVDC-link voltage output, Fig.6.3 shows the same value as expected. The results show that the simulation model operates correctly.

(a) Power Grid Voltage(RMS) (b) HVDC–link DC Voltage

Figure 6.3: Simulation Result during Normal operation

6.2.2 Case Studies

In this study, four diﬀerent cases are simulated to verify the validity of the proposed control strategy. The transient analysis in all cases is performed during an AC main grid fault to check what phenomenon occurs. The physical model in the simulation is shown in Fig. 6.4. A single-phase fault or 3-phase fault are triggered at 4.5 seconds and the duration is 0.15 seconds in the AC main grid. The simulation scenario is outlined in Table.6.3. In the AC main grid, phase-A voltage drops to zero due to phase-A fault as shown in Fig.6.5. As reviewed in Chapter 7, the HVDC-link voltage rises as shown in Fig.6.6. The rise of the DC voltage clearly illustrates the over-voltage problem that 48 occurs on the DC-link in the event of an AC main grid fault. This indicates that excessive over-voltages may occur on the DC-link if the proper control is not enabled.

DC 640kV Terminal2 AC Terminal1 230/33kV Main Grid 100km Y Y 230/370kV PMSG - 450MW 10km (Wind Farm 1) Y

230kV 230/33kV Onshore Oﬀshore Y Y 10km PMSG - 450MW MMC-VSC MMC-VSC (Wind Farm 2)

Figure 6.4: Simulation model-single phase fault

Table 6.2: Simulation cases

Simulation case Description

Case 1 Natural response(without any control methods) Case 2 Reduction of Wind power output at the generator level Case 3 Application of a DC chopper Case 4 Proposed method

Table 6.3: Transient simulation scenario Condition Description AC main Grid Fault Location (Connected to onshore converter) a 1-phase ground fault(A phase) Fault Type b 3-phase fault Fault Start 4.5 sec Fault End 4.65sec (duration : 0.15s) 49 6.3 Simulation Results

6.3.1 Case 1: Natural response(without any control method)

Fig.6.5 shows the voltage shape of the AC main grid when a 1-phase ground or 3-phase fault occurs on the AC main grid. When a single ground fault occurs on the main grid phase-A, the voltage of the corresponding phase becomes nearly zero as shown in Fig.6.5(a), and the RMS value of the overall 3-phase voltage decreases rapidly as seen in Fig.6.5(b). In the case of a 3-phase fault, the voltage decreases more signiﬁcantly as shown in Fig.6.5 (c) and (d).

Fault Fault occurs Occurs

(a) Onshore grid Aφ Voltage-1phase fault (b) Onshore grid 3φ(RMS)-1phase fault

Fault Occurs Fault Occurs

Figure 6.5: AC main grid voltage during transient analysis

Fig.6.6. shows the proﬁle of DC-link voltage in the case of a 1-phase or 3-phase fault in the onshore AC main grid. If no control strategy is applied in the event of a fault at 4.5 seconds, the DC-link voltage increases signiﬁcantly. As shown in Fig.6.6 (c), a larger rise of the DC voltage occurs at the 3-phase fault than at the 1-phase fault. 50

Fault Occurs Fault Occurs

(a) HVDC-link voltage during 1φ fault (b) HVDC-link voltage during 3φ fault

3 phase fault

1 phase fault

Fault Occurs

Figure 6.6: HVDC-link DC voltage during transient analysis

As reviewed in Chapter 5, the reason for DC-link voltage rise can be explained through the simulation results of AC main grid power and Wind grid power as shown Fig.6.7 and Fig.6.8. Wind power generation continues to produce the same amount of power, 900MW during a fault, however, power on the AC main grid is signiﬁcantly reduced as shown in Fig.6.5 and Fig.6.8. Hence, it can be seen that the surplus power is accumulated in the DC-link and the DC-link voltage rises since the power produced at the oﬀshore wind farm cannot be transferred to the AC main grid. 51

Fault Fault Occurs Occurs

(a) Wind power during 1φ fault (b) Wind power during 3φ fault

Figure 6.7: Wind power status during transient analysis

Fault Occurs Fault Occurs

(a) Onshore grid power during 1φ fault (b) Onshore grid power during 3φ fault

Figure 6.8: AC main grid Power during transient analysis

In order to examine the mechanical and electrical stresses on the wind turbine side, changes in the power in the wind turbine side and the machine speed and electric torque of the generator are also analyzed. Fig.6.9 shows the conﬁguration diagram of the wind turbine side. The wind speed is set at a constant speed of 10m/s so that mechanical changes due to wind are not considered. The wind turbine rotates the blades by the wind to produce power from the generator, and all the power generated is transferred to the external system via a 2-level VSC-converter. Fig.6.10 shows the power produced by the wind generator and the power transmitted from the converter to the oﬀshore wind farm grid. Even if a fault occurs in the main grid, there is no change in wind output power, so no change in power ensues at both locations. This shows that no mechanical and electrical stress appears in the generator. Fig.6.11 also shows the machine speed and electrical torque of the generator. The values simulated so far in this natural response of the transient study, Case 1, can be applied as a reference value to assess the FRT 52 performance when control schemes are applied.

2-Level VSC 5.2 MVA 5.0 MW DC 640kV AC Grid 100km 230/370kV PM_Gen

230kV Onshore Offshore MMC-VSC MMC-VSC

Figure 6.9: Physical Model of Wind Turbine Side

Fault Occurs Fault Occurs

(a) Wind generator side output power (b) Grid side output power

Figure 6.10: Comparison of in&output power in wind turbine converter during 1φ fault

Fault Occurs Fault Occurs

(a) Machine speed (b) Electric Torque Figure 6.11: Machine speed and electric torque in wind generator during 1φ fault 53 6.3.2 Case 2: Existing Wind Power Output control

In the event of a fault of the main grid, the wind power declines rapidly by regulating the frequency, voltage or reference power of the wind farm grid based on the communication line between converter stations [6][7][11][12]. Case 2 simulates the case of reducing wind power by controlling the reference power based on the suggestion from the existing research method. Wind turbines reduce generator power through generator side converter control. In the case of a fault, the AC main grid power is reduced to zero as shown in Fig.6.5. The HVDC-link voltage increases due to the power reduction of the onshore HVDC converter. If the HVDC-link voltage increases above the threshold as shown in Fig.5.5, the wind turbine converter reduces the power of the generator. It detects the rise of DC-link voltage by using communication lines between HVDC converter stations. Reduced generator power decreases DC-link voltage and shortens the recovery time as shown in Fig.6.12 and Fig.6.13.

Fault Occurs

Figure 6.12: The HVDC-link voltage via wind power reduction - 1φ fault

This result is due to the reduction of wind power output, which curtails the power ﬂowing into the DC-link. Reduction in wind power can help reduce the DC link voltage and recovery time. However, as the control of the wind turbine is involved, Case 2 shows that the output power of the generator and the wind turbine converter are signiﬁcantly 54

Fault Occurs

Figure 6.13: The HVDC-link voltage via wind power reduction - 3φ fault reduced compared to the results of Case 1 in which no control is employed as shown in Fig.6.14 and Fig.6.15. This may cause mechanical and electrical stress on the wind turbine. This can also be seen from changes in the generator’s machine speed and electrical torque as shown in Fig.6.16 and Fig.6.17.

Fault Occurs

Figure 6.14: Wind generator side output power-Case 1 and Case 2 during 1-phase fault 55

Fault Occurs

Figure 6.15: Grid side output power - Case 1 and Case 2 during 1-phase fault

Fault Occurs

Figure 6.16: Machine speed in wind generator - Case 1 and Case 2 during 1-phase fault 56

Fault Occurs

Figure 6.17: Electrical torque in wind generator - Case 1 and Case 2 during 1-phase fault

6.3.3 Case 3: Application of a DC chopper

Fig.6.18 and Fig.6.19 show the voltage proﬁle of the DC-link when applying a DC chopper for the 1-phase and 3-phase faults, respectively. With the operation of the DC chopper, the DC-link voltage is maintained at a fast predetermined level. DC Chopper operates by detecting over-voltage through a DC-link voltage measurement. However, chopper operation can cause unstable oscillation in the DC grid [7]. As shown in Fig.6.18 and Fig.6.19, signiﬁcant oscillation occurs until reaching a predetermined voltage value. 57

Fault Occurs

Figure 6.18: The HVDC-link voltage via DC Chopper - 1-phase fault

Fault Occurs

Figure 6.19: The HVDC-link voltage via DC Chopper - 3-phase fault

Fig.6.20 shows the current and dissipated power ﬂowing through the chopper resistor in case of a fault with 150 ms duration. As shown in Fig.6.20, the excess power dissipates quickly in the chopper resistor, but this operation characteristic may lead to a large ambiguous ﬂuctuation in the DC grid. 58

Fault Fault Occurs Occurs

(a) Current via chopper resistor (b) Dissipated power via chopper resistor Figure 6.20: Chopper circuit to dissipate excessive power

6.3.4 Case 4: Proposed method

Using the proposed method, a single phase fault with 150 ms duration in the AC main grid is simulated and the result is shown in Fig.6.21. Fast fault detection is achieved through the proposed algorithm. Then, through the current control of the main controller and wind turbine converter, it quickly recovers to a predetermined DC-link voltage without DC chopper operation. Fig.6.22 is the result of a comprehensive comparison, including the three cases reviewed. The proposed method shows that the voltage vari- ation is less and the voltage recovery time is faster than the existing methods. Fig.6.23 shows the DC-link voltage with the proposed method in the case of a main grid 3-phase fault. A 3-phase fault rarely occur in a grid system, and there is a limited chance of a fault if the transmission tower collapses or the three-phase cables are damaged. In this case, the DC-link voltage can be recovered through cooperative control with the previously employed DC chopper since it has a profound eﬀect on the system as shown in Fig.6.23. Fig.6.24 shows the result of comprehensive simulation results of the four cases analyzed so far. The proposed method shows that it is somewhat superior in over-voltage control and recovery characteristics through cooperative control than Case 3, which is only applied by a DC chopper. 59

Fault Occurs

Figure 6.21: The DC-link voltage via Proposed method - 1-phase fault

Fault Occurs

Figure 6.23: The DC-link voltage via Proposed method - 3-phase fault 60

Fault Occurs

Figure 6.22: The comprehensive analysis of DC-link voltage in 4 cases - 1-phase fault

Fault Occurs

Figure 6.24: The comprehensive analysis of DC-link voltage in 4 cases - 3-phase fault 61 When the proposed method is applied, the output power of the wind turbine side is compared, as previously reviewed, to evaluate the mechanical and electrical stress of the wind turbine. As shown in Fig.6.25, there is no signiﬁcant change in the output power of the wind generator, but the power output to the grid is notably reduced through the control of the wind turbine converter. Through this method, the wind power can be reduced while minimizing the impact on the wind generator. In the machine speed and electrical torque of the wind generator, a slight change occurs compared to Case 1 without the control scheme as shown in Fig.6.26. This demonstrates that the mechanical and electrical stress of the wind turbine is minimized when the proposed method is applied.

Fault Occurs Fault Occurs

(a) Wind generator side output power (b) Grid side output power

Figure 6.25: Comparison of output power in wind turbine during 1-phase fault

Fault Occurs

(a) Machine speed (b) Electric Torque

Figure 6.26: Machine speed and Electric torque in wind generator during 1-phase fault

Fig.6.27 through Fig.6.30 show the results of a comprehensive comparison of the cases discussed above for the wind generator stress. The DC chopper method, which is Case 3, is omitted since the results are the same as in Case 1, where the control scheme is not applied. 62

Fault Occurs

Figure 6.27: The comparison of wind generator side output power in 3 cases - 1-phase fault

Fault Occurs Case1:Natural Response

Case4:Proposed Method

Case2:Wind power Reduction

Figure 6.28: The comparison of grid side output power in 3 cases - 1-phase fault 63

Case2:Wind power Reduction

Case1:Natural Response

Fault Case4:Proposed Method Occurs

Figure 6.29: The Comparison of Wind generator’s Machine speed in 3 cases - 1-phase fault

Fault Occurs

Figure 6.30: The Comparison of Wind generator’s Electric Torque in 3 cases - 1-phase fault Chapter 7

Conclusions and Future Work

7.1 Conclusions

In this paper, a MMC-HVDC connected to a large wind farms is investigated and strategies for improving fault ride-through capability are presented. The application of HVDC is inevitable in offshore wind farms requiring long-distance, large-capacity transmission. In the event of a fault in the main grid, large offshore wind farms must meet the LVRT requirements to maintain the grid connection with the rate of voltage drop and time. As a result, the operation of wind power generators continues and the generated power cannot be delivered to the grid, which inevitably raises the voltage of the HVDC link due to the accumulation of surplus power. Therefore, a control algorithm is proposed in this paper to solve this problem. The control strategy focuses on fast fault detection, minimal application of DC choppers, and minimization of mechanical and electrical stresses of the generator. The simulation results show that large voltage oscillation is alleviated, fast recovery time and stable maintenance of the DC-link voltage are possible in the event of a 1-phase fault. In the case of a critical fault of the system such as 3-phase fault, FRT performance could be improved by suggesting a cooperative control method with the existing DC chopper. This method also has the advantage of detecting faults quickly even when an unexpected communication delay occurs due to long-distance communication between converter stations, and are able to contribute to reducing the cost of cooling equipment by minimizing heat generation. To verify the effectiveness of the control strategies, the PSCAD/EMTDC program is used to model

64 65 900MW PMSG wind farms connected to MMC-HVDC systems and 1-phase and 3-phase faults in the main system are simulated.

7.2 Future Work

In this paper, a study on control strategies in case of the main grid fault was conducted, but the following additional studies can be investigated further.

• In many cases, multiple oﬀshore wind farms are conﬁgured in the form of multi- terminals. Therefore, in this case, a study of a control strategy to satisfy LVRT is needed.

• When various types of wind power generators are applied, it is necessary to establish a control strategy suitable for them.

• For various types of faults as well as a main grid fault, studies that reﬂect their characteristics are needed.

• This study should be presented in parallel with a study on the protection scheme for large wind farms connected to the MMC-HVDC system. References

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