SCIG Based Wind Energy Integrated Multiterminal MMC-HVDC

sustainability

Article

SCIG Based Wind Energy Integrated MultiterminalMMC-HVDC Transmission Network

Md Ismail Hossain 1 and Mohammad A. Abido 1,2,*1 Electrical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31216,

Saudi Arabia; [email protected] K.A. CARE, Energy Research & Innovation Center (ERIC), Dhahran 31216, Saudi Arabia* Correspondence: [email protected]; Tel.: +966-508757838

Received: 5 April 2020; Accepted: 26 April 2020; Published: 30 April 2020

Abstract: Modular multilevel converter (MMC) based HVDC system for renewable energy integrationhas attracted the researcher’s interest nowadays. This paper proposes a control strategy for MMC basedmultiterminal HVDC system for grid integration of squirrel cage induction generator (SCIG) basedwind energy systems. Unlike the average model, this work models the MMC using the aggregate modeland develops multiterminal HVDC transmission network in MATLAB/Simulink. It further developsthe MMC multiterminal HVDC transmission network in real time digital simulator (RTDS). Insteadof simplified current source, the proposed network considers the complete dynamics of SCIG basedwind source from generation to integration. It employs field-oriented control for optimum wind energytracking and forms isolated AC grids using feed forward controller. The proposed MMC controllerperformance has been tested under severe balanced and unbalanced disturbances. The results fromaggregate model based MMC network in MATLAB/Simulink and those of the experimental MMCnetwork in RTDS are in full agreement. The results confirm optimum wind energy tracking, fulfill gridcode requirements, and improve low voltage ride through capability.

Keywords: MMC aggregate model; MMC detailed model; field-oriented control; multiterminalHVDC; balanced fault; unbalanced fault; SCIG

1. Introduction

Owing to reactive power support, black start capability, smaller filter size, and power reversalwithout changing, the voltage polarity of the DC-link voltage makes the VSC-HVDC system suitable foroffshore and onshore renewable energy integration over LCC-HVDC system [1,2]. Modular multilevelconverter (MMC) based HVDC system for renewable energy integration has become popular aroundthe world because of its modular characteristics and low switching frequency operation [3,4]. Besides,MMC reduces the harmonics distortion by generating almost pure sine waves, which does not requireexpensive filters at the point of common coupling of the AC grids. The interaction of MMC based HVDCline with AC overhead line has been studied in [5]. The paper in [6] discussed the multiterminal HVDCcontrol strategy during DC fault. However, balancing capacitor voltages of the submodule and armcirculating currents have not been considered in [5] and [6]. Apart from this, the system performancehas not been tested in real time simulation. The literature in [7,8] have discussed power dependentdroop and distributed DC voltage based control techniques of multiterminal HVDC grids where theaverage model has been used in HVDC simulation. A large number of literature have been reportedfor wind energy integration to the VSC HVDC system using permanent magnet synchronous generator(PMSG) and doubly-fed induction generator (DFIG). However, no paper addressed the squirrel cageinduction generator based wind energy integration into the MMC-HVDC system. Over the decades,the cost of semiconductor switch has significantly dropped and converter design has been improved.

Sustainability 2020, 12, 3622; doi:10.3390/su12093622 www.mdpi.com/journal/sustainability

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This progress has made the full scale converter based type-4 wind generators competitive against type-3wind generators. Besides, it can easily support the low voltage ride-through and fulfill the strictestgrid code requirements. Stator terminal of DFIG is directly connected to grid which complicates thegrid code requirements. However, PMSG is expensive and requires rare earth materials for its magnet.Only a few countries have such materials and could restrict its flow [9–12]. Due to simplicity, reliability,ruggedness, less maintenance, quiet operation, low cost, high performance, and longevity, inductionmachine placed a dominant role in different kinds of industry. Therefore, SCIG with full scale convertercould be potential wind generators for offshore or onshore platform.

The multiterminal DC grids with solar and wind are proposed in [13–16], where complete dynamicsof PV/wind integration are ignored and presented by a simplified current source. The work reportedin [17,18] presented a multiterminal HVDC grids with induction generator, and considered two-levelconverter for the HVDC grids. Work in [18] used single VSC to control a whole wind farm. However,this approach is not practical and unable to track individual optimum points. Instead of wind farm,a small system with one induction generator connected through single VSC is presented in [17].

The short circuit analysis of the offshore AC network connected through multiple VSC-HVDClinks was discussed [19]. Detailed analysis for a single line to a ground fault has been carried out [20].However, no results were presented from the detailed model or real time hardware simulation in [19]and [20]. During faults on the AC side of MMC, the power transfer capability of the converter becomeslimited, which in turn increases the DC link voltage. Apart from the dynamic braking resistor [21,22],the works in [23–26] suggested solution that requires fast communication among different stations,coordination among converter stations for automatic adjustment based on HVDC voltage as thereference signal and centralized control for DC link voltage control during fault in the point of commoncoupling. This work employs dynamic braking resistor based local control for DC link overvoltageprotection. Also, a control strategy is developed where DC link voltage is given more priority overfixed reactive power control. During three-line to ground fault, fixed reactive power control is replacedwith maximum reactive power injection control.

The works in [27–29] have used capacitor and its dynamics to form isolated MMC based ACnetworks for offshore wind integration into the HVDC network. However, MMC based HVDCsystem generates a small number of harmonics, which is acceptable without having a filter. Besides,the capacitor is placed on the high voltage AC side, which in turn absorbs high reactive power.

This work adopts the aggregate model of MMC to capture harmonics dynamics and circulatingarm current control to model the MMC in MATLAB Simulink. However, a detailed model of MMCis used in real time digital simulation (RTDS), where low-level control is considered. Furthermore,it includes the complete dynamics of one renewable energy unit instead of a simplified current sourceand then scaled up its capacity by replicating the behavior of one SCIG unit. In this work, an isolated ACgrids for wind energy integration is formed employing feed forward controller. It further investigatesthe controller performance in line with the grid code requirements during severe balance (under/overvoltage, three phases to ground solid fault) and unbalanced solid fault at the point of common couplingof MMC to improve the fault ride through capability of the converter. Besides low voltage ridethrough, it evaluates the controller performance during under and over frequency at the point ofcommon coupling of MMC. This work also compares the result from the aggregate model basedMMC in MATLAB Simulink with the detailed model based MMC in RTDS. To the best of the authors’knowledge, such an approach has not been reported yet in the literature. The following shortcomingsin current studies are to be addressed as the main contributions of this research:

(a) Average model was considered while modeling MMC in HVDC network;(b) Detailed model of MMC with real time simulation results were not presented;(c) SCIG based wind energy integrated MMC-HVDC system was not reported;(d) Simplified current source instead of complete dynamics of renewable energy was considered;(e) The effect of different kinds of faults on the DC link voltage and power flow with grid code

requirements were not properly addressed;


(f) Capacitor was placed on the high voltage AC side while forming isolated AC grids for windenergy integration.

The rest of the paper is organized as follows. Section 2 provides the mathematical modeling andcontroller design of the overall system. Section 3 presents the simulation results along with necessarydiscussions, whereas Section 4 draws the conclusions. Finally, the Acknowledgement, Nomenclature,and References are appended at the end of the article.

2. System Overview and Controller Design

The multiterminal HVDC network is presented in Figure 1. The system consists of a wind farmconnected through MMC4 terminal whereas the rest MMCs are connected with AC grids. MMC4terminal creates the isolated AC network to integrate the wind energy. The squirrel cage inductiongenerator side VSC ensures the maximum tracking of wind energy during wind change. Grid side VSCcontrols the DC link voltage and transfers all energy injected by wind generator to MMC4 supportedAC grids. The power injected by the detailed model based one unit of wind generator at the point ofcommon coupling (PCC) is scaled to 100 MW by multiplying by 50. Instead of simplified equivalentcurrent source for whole wind farm, this process incorporates the dynamics of one renewable energyunit. MMC1 controls the DC link voltage, whereas MMC2 and MMC3 work as real and reactive powercontroller. The charging resistor, Rlimit is used to limit the in-rush current during start-up process. At acertain point, the voltage on the DC side is approximately equal to the peak value of the line-to-linevoltage of AC side. At this point, when the DC side is charged, the submodule capacitors in all arms arealso charged. Simultaneously, the charging resistors Rlimit is short-circuited by means of a mechanicalcircuit breaker. After that the MMC1 gradually controls the HVDC link voltage to its rated value.Usually, wind generator voltage is low and practical single stage transformer cannot provide requiredtransformation ratio to connect with HVDC line. Therefore, the voltage of wind farm at the point ofcommon coupling (PCC) is stepped up by two stages transformer to match the high power and highvoltage rating of HVDC transmission network.

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(e) The effect of different kinds of faults on the DC link voltage and power flow with grid code

requirements were not properly addressed;

(f) Capacitor was placed on the high voltage AC side while forming isolated AC grids for wind energy integration.

The rest of the paper is organized as follows. Section II provides the mathematical modeling and controller design of the overall system. Section III presents the simulation results along with necessary discussions, whereas Section IV draws the conclusions. Finally, the Acknowledgement, Nomenclature, and References are appended at the end of the article.

2. System Overview and Controller Design

The multiterminal HVDC network is presented in Figure 1. The system consists of a wind farm connected through MMC4 terminal whereas the rest MMCs are connected with AC grids. MMC4 terminal creates the isolated AC network to integrate the wind energy. The squirrel cage induction generator side VSC ensures the maximum tracking of wind energy during wind change. Grid side VSC controls the DC link voltage and transfers all energy injected by wind generator to MMC4 supported AC grids. The power injected by the detailed model based one unit of wind generator at the point of common coupling (PCC) is scaled to 100 MW by multiplying by 50. Instead of simplified equivalent current source for whole wind farm, this process incorporates the dynamics of one renewable energy unit. MMC1 controls the DC link voltage, whereas MMC2 and MMC3 work as real and reactive power controller. The charging resistor, Rlimit is used to limit the in-rush current during start-up process. At a certain point, the voltage on the DC side is approximately equal to the peak value of the line-to-line voltage of AC side. At this point, when the DC side is charged, the submodule capacitors in all arms are also charged. Simultaneously, the charging resistors Rlimit is short-circuited by means of a mechanical circuit breaker. After that the MMC1 gradually controls the HVDC link voltage to its rated value. Usually, wind generator voltage is low and practical single stage transformer cannot provide required transformation ratio to connect with HVDC line. Therefore, the voltage of wind farm at the point of common coupling (PCC) is stepped up by two stages transformer to match the high power and high voltage rating of HVDC transmission network.

Rlimit AC grid 1

MMC4

∆- Y

WIND FARM (50×2MW=100MW)

Vac4

PCC

SCIGGear box

Turbine

DBR

PCC1MMC1

Rlimit MMC2

Rlimit MMC3

∆- Y Y-∆

Y-∆

∆- Y

AC grid 2 AC grid 3

MT-

HVDC

200k

V

R-L

690V-22kV

22kV-122.47kV

460kV-100kV

Vdc

100kV-460kV

100kV-460kV

GSC MSC

IsC

VPCC

IPCCRF

Figure 1. Wind energy integrated multiterminal HVDC network.

2.1. Wind Generator Side Converter Control for Optimum Wind Energy Integration

The mechanical power of a wind turbine is given by the following Equation [30] = 0.5 ( , ) (1)

Figure 1. Wind energy integrated multiterminal HVDC network.

2.1. Wind Generator Side Converter Control for Optimum Wind Energy Integration

The mechanical power of a wind turbine is given by the following Equation [30]

Pturbine = 0.5ρAV3wCp(λ, β) (1)


where, A is the turbine swift area, ρ is the air mass density, Vw is the wind speed and Cp(λ, β) is calledthe performance coefficient, which is dependent on the blade angle, β and the tip speed ratio λ. The tipspeed ratio is defined as

λ =rωturbine

Vw(2)

where r is the turbine radius, and ωturbine is the turbine angular speed.Performance coefficient is defined as [31]

Cp(λ, β) =12

(λ− 0.022β2

− 5.6)e−0.17λ (3)

Apart from the blade angle, it is inferred from Equation (1) to Equation (3) that the turbine speedis the only controlling parameter as the wind speed cannot be changed. Turbine speed or generatorspeed controls the tip speed ratio that eventually changes the turbine power. Now the turbine speed isgoverned by the interaction between turbine torque and machine electrical torque. Figure 2 shows theturbine mechanical power versus the generator speed at different wind speeds. The optimum point ofturbine mechanical energy changes with the change of wind speed. Therefore, the fixed speed of thegenerator is not capable of extracting optimum wind energy at different wind speeds.


where, A is the turbine swift area, 𝜌 is the air mass density, 𝑉𝑤 is the wind speed and 𝐶𝑝(𝜆, 𝛽) is

called the performance coefficient, which is dependent on the blade angle, 𝛽 and the tip speed ratio

𝜆. The tip speed ratio is defined as

𝜆 = 𝑟𝜔𝑡𝑢𝑟𝑏𝑖𝑛𝑒

𝑉𝑤 (2)

where r is the turbine radius, and 𝜔𝑡𝑢𝑟𝑏𝑖𝑛𝑒 is the turbine angular speed.

Performance coefficient is defined as [31]

𝐶𝑝(𝜆, 𝛽) =1

2(𝜆 − 0.022𝛽2 − 5.6)𝑒−0.17𝜆 (3)

Apart from the blade angle, it is inferred from Equation (1) to Equation (3) that the turbine speed is

the only controlling parameter as the wind speed cannot be changed. Turbine speed or generator

speed controls the tip speed ratio that eventually changes the turbine power. Now the turbine speed

is governed by the interaction between turbine torque and machine electrical torque. Figure 2 shows

the turbine mechanical power versus the generator speed at different wind speeds. The optimum

point of turbine mechanical energy changes with the change of wind speed. Therefore, the fixed speed

of the generator is not capable of extracting optimum wind energy at different wind speeds.

Figure 2. Wind turbine characteristics.

The converter employs the field-oriented control technique to control the generated electrical

torque set by the optimum point of wind energy, which forces the machine to run that speed

corresponding to the optimum point of wind energy. Field oriented control technique is able to

independently control the electrical torque and rotor speed of the SCIG.

The machine d-q current control dynamics in rotor-field coordinate is given by the following

Equation [30]

𝜎𝜏𝑠

𝑑𝑖𝑑

𝑑𝑡+ 𝑖𝑑 = 𝜎𝜏𝑠𝜔𝑖𝑞 +

𝑉𝑑

𝑅𝑠 (4)

𝜎𝜏𝑠

𝑑𝑖𝑞

𝑑𝑡+ 𝑖𝑞 = −𝜎𝜏𝑠𝜔𝑖𝑑 − (1 − 𝜎)𝜏𝑠𝜔𝑖𝑚𝑟 +

𝑉𝑞

𝑅𝑠

(5)

Field oriented control works based on the position of rotor flux, which is used to convert

stationary stator current to rotational d-q current. The angular speed of rotor flux is given below.

𝜔 = 𝜔𝑟 +𝑖𝑞

𝜏𝑟𝑖𝑚𝑟 (6)

Figure 2. Wind turbine characteristics.

The converter employs the field-oriented control technique to control the generated electrical torqueset by the optimum point of wind energy, which forces the machine to run that speed corresponding tothe optimum point of wind energy. Field oriented control technique is able to independently controlthe electrical torque and rotor speed of the SCIG.

The machine d-q current control dynamics in rotor-field coordinate is given by the followingEquation [30]

στsdiddt

+ id = στsωiq +VdRs

(4)

στsdiqdt

+ iq = −στsωid − (1− σ)τsωimr +Vq

Rs(5)

Field oriented control works based on the position of rotor flux, which is used to convert stationarystator current to rotational d-q current. The angular speed of rotor flux is given below.

ω = ωr +iq

τrimr(6)


The integration of Equation (6) gives the rotor flux position. The relation between d-axis currentand magnetizing current is governed by the following equation.

τrdimr

dt+ imr = id (7)

The generated electrical torque is

Te =3Lm

2(1 + σr)imriq (8)

Equation (8) indicates that the electrical torque proportionally changes with the q-axis currentas magnetizing current mostly remains constant. SCIG draws the magnetizing current when it isconnected to the AC source.

The magnetizing current is given by the following equation.

imr_re f = id_re f =

√23

Vline to line rms

(1 + σs)Lmω0(9)

Stator time constant, τs =(1 + σs)

RsLm (10)

Rotor time constant, τr =(1 + σr)

RrLm (11)

Stator leakage factor, σs =Ls

Lm− 1 (12)

Rotor leakage factor, σr =Lr

Lm− 1 (13)

Machine total leakage factor, σ = 1−1

(1 + σr)(1 + σs)(14)

Ls, Lr and Lm are the stator, rotor and mutual inductance of the SCIG respectively. The referencetorque is found from the wind turbine characteristics curve, as shown in Figure 2, and then convertedto q-axis reference current using Equation (8). Equation (7) is used to provide reference d-axis current.Finally, Equations (4) and (5) produce converter d-q axis voltage that further generates the modulatingsignal for the converter. Figure 3 illustrates the field oriented control technique based wind generatorside converter control.


The integration of Equation (6) gives the rotor flux position. The relation between d-axis current

and magnetizing current is governed by the following equation. + = (7)

The generated electrical torque is = 32(1 + ) (8)

Equation (8) indicates that the electrical torque proportionally changes with the q-axis current as magnetizing current mostly remains constant. SCIG draws the magnetizing current when it is connected to the AC source.

The magnetizing current is given by the following equation.

_ = _ = 23 (1 + ) (9)

Stator time constant, = ( ) (10)

Rotor time constant, = ( ) (11)

Stator leakage factor, = − 1 (12)

Rotor leakage factor, = − 1 (13)

Machine total leakage factor, = 1 − ( )( ) (14)

Ls, Lr and Lm are the stator, rotor and mutual inductance of the SCIG respectively. The reference torque is found from the wind turbine characteristics curve, as shown in Figure 2, and then converted to q-axis reference current using Equation (8). Equation (7) is used to provide reference d-axis current. Finally, Equations (4) and (5) produce converter d-q axis voltage that further generates the modulating signal for the converter. Figure 3 illustrates the field oriented control technique based wind generator side converter control.

abc to dq

Rotor Shaft Encoder

+ ωs

ωrω

θ IS

Imr-ref

Id

Iq

1τrs+1 ÷

×

Lm

+

-Imr

PI11

Id-ref

× ÷

Te-ref

2(1+σr)

3Lm

Iq-ref

+ PI1

-Iq-ref

Iq

+

×

×

στs Id

ω

(1-σ)τs Imr

ω

Rs

+ PI1- Id

+

× στs Iq

ω

Rs

dq to

abc

vq

md mq

θ

×÷

vd

÷ ×

vd

vq

vdc 2

Real

Imag

Mag

Angle

Mag

Angle

Real

Imag

Limiter md

mq

3-phase current

-abc to PWM gate pulse

MSC

Figure 3. Wind generator side converter control. Figure 3. Wind generator side converter control.


2.2. Grid Side Converter Control

The purpose of the grid side VSC is to control the DC link voltage and transfer the wind energyinjected by the machine side converter to the PCC terminal. Besides the DC link voltage controller,the crowbar controller, which is a switch with a series resistor (RF), protects the DC link capacitor fromovercharge if the grid side converter fails to transfer wind energy during the fault. The followingequation governs the dynamics of inner current controller in d-q frame in the steady state [30].

Ldisddt

+ Risd = Lω0isq + Vd −Vsd (15)

Ldisq

dt+ Risq = −Lω0isd + Vq −Vsq (16)

The dynamics of the DC link voltage controller is governed by the following equation in thed-q frame

CdVdc

dt= Idclink − isd (17)

Equation (15) to Equation (17) form the basis of the outer and inner control loop of the AC gridsside VSC, supported by MMC4. Figure 4 shows the DC link voltage controller that processes theDC link voltage control error through a PI controller and provides a reference current, Isd-ref to theinner current control loop. Reactive power is set to zero for transferring all wind energy into theMMC4 terminal.


2.2. Grid Side Converter Control

The purpose of the grid side VSC is to control the DC link voltage and transfer the wind energy injected by the machine side converter to the PCC terminal. Besides the DC link voltage controller, the crowbar controller, which is a switch with a series resistor (RF), protects the DC link capacitor from overcharge if the grid side converter fails to transfer wind energy during the fault. The following equation governs the dynamics of inner current controller in d-q frame in the steady state [30]. + = + − (15)

+ = − + − (16)

The dynamics of the DC link voltage controller is governed by the following equation in the d-q frame = − (17)

Equation (15) to Equation (17) form the basis of the outer and inner control loop of the AC grids side VSC, supported by MMC4. Figure 4 shows the DC link voltage controller that processes the DC link voltage control error through a PI controller and provides a reference current, Isd-ref to the inner current control loop. Reactive power is set to zero for transferring all wind energy into the MMC4 terminal.

abc to dq

ω0

Isd-sq

Qgref Isq-ref

dq-abc mdq

×÷ ÷ ×

vd

vq

vdc 2

Real

Imag

Mag

Angle

Mag

Angle

Real

Imag

Limiter md

mq

θg

abc to dq Vsd-sqPLLVPCCθg

θg

θg Vdc-ref + PI4-

Vdc

+

Iext Filter

Limiter

Isd-ref

×÷ vsd

-23

Filter

Filter

Filter

Isd-ref+ PI3

- Isd

+

× Isq

ω0

vd

-VsdIsq-ref+ PI3

- Isq

+

× Isd

ω0

vq Vsq

LL

VPCC

IPCC

abc-PWM gate pulse

GSC

Figure 4. Grid side converter control.

2.3. Controller Performance Analysis of Machine Side Converter

The performance of wind generator side converter controller for tracking the optimum wind energy has been analyzed in MATLAB Simulink. Figure 5 shows the extracted wind power by the machine side converter due to wind speed variation. The DC link power of machine side converter increases to maximum at rated wind speed. At this point the generator speed remains constant at its rated value. As can be seen from Figure 6 that the turbine torque changes instantly with the abrupt wind speed change; however, electrical torque slowly changes due to inertia. Since the converter controller response is fast, the reference electrical torque is gradually set to an optimum value with the generator speed to minimize the mechanical stress. Figure 6 points that the actual torque follows the reference torque, and the mechanical torque eventually adjusts its value corresponding to the reference electrical torque.

Figure 4. Grid side converter control.

2.3. Controller Performance Analysis of Machine Side Converter

The performance of wind generator side converter controller for tracking the optimum wind energyhas been analyzed in MATLAB Simulink. Figure 5 shows the extracted wind power by the machineside converter due to wind speed variation. The DC link power of machine side converter increases tomaximum at rated wind speed. At this point the generator speed remains constant at its rated value.As can be seen from Figure 6 that the turbine torque changes instantly with the abrupt wind speedchange; however, electrical torque slowly changes due to inertia. Since the converter controller responseis fast, the reference electrical torque is gradually set to an optimum value with the generator speedto minimize the mechanical stress. Figure 6 points that the actual torque follows the reference torque,and the mechanical torque eventually adjusts its value corresponding to the reference electrical torque.

Sustainability 2020, 12, 3622 7 of 27Sustainability 2020, 12, x FOR PEER REVIEW 7 of 27

Figure 5. Optimum wind power flow to machine side DC link for wind speed variation.

Figure 6. Optimum electrical torque reference, actual electrical and mechanical torque.

Figure 7 presents the current controller performance of wind generator side converter during

the wind speed change. Unlike the fixed capacitor for the reactive power support, the converter

provides the required reactive current. The reference currents for the converter have been generated

using the Equation (7), (8), and (9). It is seen from Figure 7 that the actual current tracks the reference

current with negligible overshoot and zero steady state error during the peak wind energy tracking.

2.4. MMC based Multiterminal HVDC Network Control

Modular multilevel converter (MMC) is the building block of the multiterminal HVDC

transmission network. It consists of hundreds of submodules to support the high power and high

voltage DC. Figure 8 presents an MMC equivalent circuit. Like other VSC, it has two control loops.

One loop is a current control loop that ensures the tracking of reference current whereas, another loop

dictates the operation type. Besides, arm circulating current and submodule voltage balancing control

are required.





Figure 7 presents the current controller performance of wind generator side converter during

the wind speed change. Unlike the fixed capacitor for the reactive power support, the converter

provides the required reactive current. The reference currents for the converter have been generated

using the Equation (7), (8), and (9). It is seen from Figure 7 that the actual current tracks the reference

current with negligible overshoot and zero steady state error during the peak wind energy tracking.


Modular multilevel converter (MMC) is the building block of the multiterminal HVDC

transmission network. It consists of hundreds of submodules to support the high power and high

voltage DC. Figure 8 presents an MMC equivalent circuit. Like other VSC, it has two control loops.

One loop is a current control loop that ensures the tracking of reference current whereas, another loop

dictates the operation type. Besides, arm circulating current and submodule voltage balancing control

are required.


Figure 7 presents the current controller performance of wind generator side converter during thewind speed change. Unlike the fixed capacitor for the reactive power support, the converter providesthe required reactive current. The reference currents for the converter have been generated using theEquations (7)–(9). It is seen from Figure 7 that the actual current tracks the reference current withnegligible overshoot and zero steady state error during the peak wind energy tracking.


Modular multilevel converter (MMC) is the building block of the multiterminal HVDC transmissionnetwork. It consists of hundreds of submodules to support the high power and high voltage DC.Figure 8 presents an MMC equivalent circuit. Like other VSC, it has two control loops. One loop is acurrent control loop that ensures the tracking of reference current whereas, another loop dictates theoperation type. Besides, arm circulating current and submodule voltage balancing control are required.


Figure 7. Performance of current controller of machine side VSC.

2.4.1. High Level Control

Outer loop control is called high-level control. It can operate as high voltage DC link and reactive

power/AC grids voltage control, real and reactive power control, and isolated AC voltage control, as

presented in Figure 8. In this work, MMC DC link voltage control and AC voltage control are given

more priority than fixed reactive power control during fault at the point of common coupling of the

AC grids. During line to ground fault where possible maximum real power transfer is limited results

more reactive power support irrespective of its reactive power command. HVDC link voltage

controller includes a PI controller that processes the error and feed forward controller (information

of HVDC link current) for improved dynamic performance. The PI controller provides the necessary

adjustment for the reactive power loop controller against its reference value. The real power

controller works the same way as the reactive power controller. Unlike works in [27–29] where

capacitor was placed on the high voltage side and decoupled controller was used, this work removes

that bulky capacitor and employs feed forward controller to form an isolated AC network for

renewable energy integration. As can be seen from Figure 8, any change in the AC network adjusts

the reference current while holding the AC voltage constant. Figure 9 illustrates that the change in

the HVDC network due to faults in the point of common coupling of AC grids is reflected on the

modulating signal to keep the AC voltage unchanged.

2.4.2. Low Level Control

The current control dynamics of half bridge MMC as presented in Figure 8 in the d-q reference

frame is given by the following equation in the steady state

𝐿

2

𝑑𝑖𝑠𝑑

𝑑𝑡+

𝑅

2𝑖𝑠𝑑 =

𝐿

2𝜔0𝑖𝑠𝑞 + 𝑉𝑑 − 𝑉𝑠𝑑 (18)

𝐿

2

𝑑𝑖𝑠𝑞

𝑑𝑡+

𝑅

2𝑖𝑠𝑞 = −

𝐿

2𝜔0𝑖𝑠𝑑 + 𝑉𝑞 − 𝑉𝑠𝑞 (19)

The HVDC link voltage controller dynamics is governed by the following equation.

(𝐶𝑑 +6𝐶

𝑁)

𝑑𝑉𝑑𝑐

𝑑𝑡= 𝐼𝑑𝑐𝑙𝑖𝑛𝑘 − 𝑖𝑠𝑑 (20)

Figure 7. Performance of current controller of machine side VSC.

2.4.1. High Level Control

Outer loop control is called high-level control. It can operate as high voltage DC link and reactivepower/AC grids voltage control, real and reactive power control, and isolated AC voltage control,as presented in Figure 8. In this work, MMC DC link voltage control and AC voltage control are givenmore priority than fixed reactive power control during fault at the point of common coupling of the ACgrids. During line to ground fault where possible maximum real power transfer is limited results morereactive power support irrespective of its reactive power command. HVDC link voltage controllerincludes a PI controller that processes the error and feed forward controller (information of HVDC linkcurrent) for improved dynamic performance. The PI controller provides the necessary adjustment forthe reactive power loop controller against its reference value. The real power controller works the sameway as the reactive power controller. Unlike works in [27–29] where capacitor was placed on the highvoltage side and decoupled controller was used, this work removes that bulky capacitor and employsfeed forward controller to form an isolated AC network for renewable energy integration. As can beseen from Figure 8, any change in the AC network adjusts the reference current while holding the ACvoltage constant. Figure 9 illustrates that the change in the HVDC network due to faults in the point ofcommon coupling of AC grids is reflected on the modulating signal to keep the AC voltage unchanged.

2.4.2. Low Level Control

The current control dynamics of half bridge MMC as presented in Figure 8 in the d-q referenceframe is given by the following equation in the steady state

L2

disddt

+R2

isd =L2ω0isq + Vd −Vsd (18)

L2

disq

dt+

R2

isq = −L2ω0isd + Vq −Vsq (19)

The HVDC link voltage controller dynamics is governed by the following equation.(Cd +

6CN

)dVdcdt

= Idclink − isd (20)


AC grid Rlimit

SM

SM

SM

SM

R

L

R

L

2C

2C

Vdc

Iext

I1

I2

V1

V2

Is

Vs

Vac

Vsd-ref+ PI7

- Vsd

+

Isd-ext

Vdc-ref+ PI4

-Vdc

+

Iext Filter

Isd-ref

Filter

Isq-ref

×÷

vsd 23

-

Qg-meas+ PI5

-+-1

Pgref

Isd-ref

×÷ vsd

23

Pg-meas+ PI6 +

ω0

Limiter0 to 2π θs

Isd-ref

Vsq-ref+ PI7

- Vsq

+

Isq-ext

Isq-ref

Filter

Filter

P or Vdc controller

Isolated Vac controller

Vacmin

Vacmax

Vac

+

+

--

PI0

+

Limiter

LimiterPI0 Vac >VacTh

0

QgrefQgref_C

YN

Qgref_C

Submodule(SM)

S1

S2

Q controller

High level controllerMMC equivalent circuit

DB

R

Figure 8. MMC equivalent circuit and high level control of MMC converter.

Figure 9 presents the inner current control loop based on Equation (18) and Equation (19). In addition to these equations, current and modulating voltage limiters are employed to prevent overcurrent through the transistor. Isd current is allowed to reach maximum permissible current due to DC link voltage control has more priority than reactive power control. However, both currents have equal weights in the real-reactive control mode of MMC if the total current exceeds the maximum permissible current (Imax). As can be seen from Figure 9, the voltage magnitude remains inside the unit circle without changing the angle of Vd and Vq. Apart from the inner current control loop, circulating current control and submodule voltage balancing control are employed in low level control. The circulating current results from the voltage difference between the upper and lower arms, does not have an impact on the AC output voltages and currents but increases the rating of the devices, power losses, and ripple in submodule capacitor voltages. The circulating current comprises twice fundamental frequency negative sequence components [32,33]. The circulating current control dynamics in the d-q reference frame rotating at 2 frequency is given by the following equation. + = 2 + (21)

+ = −2 + (22)

Figure 9 presents the arm circulating current control based on Equation (21) and (22). Since the submodule (SM) capacitor charges and discharges based on its selection and current direction, capacitor voltage drifts from one submodule to another submodule. In this work, the sorting algorithm is used to find suitable SM from a set of SMs based on the current direction. Higher voltage SM to lower voltage SM is placed if current leaves from the SM or lower voltage SM to higher voltage SM is placed if current enters to SM.

Figure 8. MMC equivalent circuit and high level control of MMC converter.

Figure 9 presents the inner current control loop based on Equations (18) and (19). In addition tothese equations, current and modulating voltage limiters are employed to prevent overcurrent throughthe transistor. Isd current is allowed to reach maximum permissible current due to DC link voltagecontrol has more priority than reactive power control. However, both currents have equal weights inthe real-reactive control mode of MMC if the total current exceeds the maximum permissible current(Imax). As can be seen from Figure 9, the voltage magnitude remains inside the unit circle withoutchanging the angle of Vd and Vq. Apart from the inner current control loop, circulating current controland submodule voltage balancing control are employed in low level control. The circulating currentresults from the voltage difference between the upper and lower arms, does not have an impact onthe AC output voltages and currents but increases the rating of the devices, power losses, and ripplein submodule capacitor voltages. The circulating current comprises twice fundamental frequencynegative sequence components [32,33]. The circulating current control dynamics in the d-q referenceframe rotating at 2ω0 frequency is given by the following equation.

Ldid1

dt+ Rid1 = 2Lω0iq1 + Vd1 (21)

Ldiq1

dt+ Riq1 = −2Lω0id1 + Vq1 (22)

Figure 9 presents the arm circulating current control based on Equations (21) and (22). Since thesubmodule (SM) capacitor charges and discharges based on its selection and current direction, capacitorvoltage drifts from one submodule to another submodule. In this work, the sorting algorithm is usedto find suitable SM from a set of SMs based on the current direction. Higher voltage SM to lowervoltage SM is placed if current leaves from the SM or lower voltage SM to higher voltage SM is placedif current enters to SM.


ω0

θ

Is

Isd

+ PI8

- Isd

+

× Isq

ω0

dq to

abc

md mq

vd

ma1

mb1

mc1

-

×÷ ÷ ×

vd

vq

vdc 2

abc to dq

Vs

Vsd

Vsq

Isq

PLL

θg

θg

θg

Vsd

Isq-ref + PI8-Isq

+

× Isd

ω0 vq

Vsq

Filter

Filter

Filter

Filter

L2

L2

Isd-ref

Imax

Imax

MIN

+(·)2

( ·)2

-√ (·)

× sign

Filter

abc to dq

Id1

-2θg

Iq1

Iupper123

+

Ilower123

0.5

+ PI10

0

+

× Iq12ω0

L

-+ PI10

0

+

× Id12ω0

L dqto

abc

-2θg

ma2

mb2

mc2

-

-

ma,b,c1

+

+-

Limiter

Limiter

-0.5 +

+0.5

0.5

N

N

UpperSMs1,2,3

LowerSMs1,2,3

ma,b,c2

Inner current control Nearest level modulation

Arm circulating current control

Figure 9. Low-level control of MMC converter.

Nearest level modulation, as shown in Figure 9 is used to generate a number of submodule selection from the combination of inner current and circulating current control signals which is further processed through the SM sorting algorithm to finalize the specific SM selection.

2.4.3. MMC Aggregate Model

Detail model of MMC is computationally expensive to simulate in MATLAB Simulink. The average model uses a pure sine wave, which ignores converter harmonics and circulating currents phenomena of MMC. However, the aggregate model preserves control system dynamics, converter harmonics, and circulating currents phenomena. In this approach, MMC is modeled using a switching-function model where only one equivalent module is used to represent all submodules of the upper or lower arm. However, this model does not include the submodule voltage unbalancing dynamics.

One submodule capacitor voltage of the aggregate model, as shown in Figure 10 is given by the following equation VC = 1CMN (I N + I N )dt + V _ (23)

Where,V = NM VC , V = N VC , Vcap_initial = submodule capacitor initial voltage, CM = submodule capacitance, Nmodule=Number of active submodule, Nblock=Number of block submodule, N=Number of submodule in upper or lower arm.

Figure 9. Low-level control of MMC converter.

Nearest level modulation, as shown in Figure 9 is used to generate a number of submoduleselection from the combination of inner current and circulating current control signals which is furtherprocessed through the SM sorting algorithm to finalize the specific SM selection.

2.4.3. MMC Aggregate Model

Detail model of MMC is computationally expensive to simulate in MATLAB Simulink. The averagemodel uses a pure sine wave, which ignores converter harmonics and circulating currents phenomenaof MMC. However, the aggregate model preserves control system dynamics, converter harmonics,and circulating currents phenomena. In this approach, MMC is modeled using a switching-functionmodel where only one equivalent module is used to represent all submodules of the upper or lowerarm. However, this model does not include the submodule voltage unbalancing dynamics.

One submodule capacitor voltage of the aggregate model, as shown in Figure 10 is given by thefollowing equation

VCapacitance =1

CMN

∫(IinNmodule + IblockNblock)dt + Vcap_initial (23)

where,

Vin = NModuleVCapacitance,Vblock = NblockVCapacitance,Vcap_initial = submodule capacitor initial voltage,CM = submodule capacitance,Nmodule = Number of active submodule,Nblock = Number of block submodule,N = Number of submodule in upper or lower arm.


Figure 10. Aggregate model of upper/lower arm of MMC Bridge.

In this way, the lower or upper arm MMC of each phase shows the capacitance voltage that is governed by the Equation (23). Unlike average model, aggregate model also facilitates the blocking option of MMC during simulation. Current flow from point A to point B could be stopped, if the Nblock

is set equal to N. Similarly, the combination of Nmodule and Nblock yields the current flow from zero to maximum. In addition, it is possible to gradually charge the submodule capacitor during startup which otherwise could draw the very high charging current. This feature points the progressively setting up the HVDC link voltage. In summary, this model allows to analyze the performance of MMC based multiterminal HVDC network during normal and fault condition using relatively inexpensive computational device without leaving most of its dynamics.

3. RTDS Simulation and Discussion

RTDS is a high computing device and works as real time simulator which facilitates the interaction with the physical hardware. It uses only fixed step discrete solver, whereas non-real time variable step solver and continuous model could be used in MATLAB Simulink. The sample time for power electronics switch is 2.3 µs and for controller is 50 µs. We have used a multirack RTDS platform to simulate such a large system considering detailed MMC model in real time. PB5 and Nova core form the multirack RTDS hardware platform. MMC1, MMC2, and MMC3 are placed in one rack, and MMC4 with wind farm are placed in another rack.

One wind generator of 2 MW with its optimum controller is developed and then scaled to 100 MW by imitating the behavior of one unit. The runtime RTDS RSCAD interface is presented in Figure 11 with running data. Table 1 and 2 provide the detailed parameters for the complete system.

Figure 11. RTDS run time interface of the complete system.

Vin

Vblock

IinIblock

A

B


In this way, the lower or upper arm MMC of each phase shows the capacitance voltage that isgoverned by the Equation (23). Unlike average model, aggregate model also facilitates the blockingoption of MMC during simulation. Current flow from point A to point B could be stopped, if the Nblock

is set equal to N. Similarly, the combination of Nmodule and Nblock yields the current flow from zeroto maximum. In addition, it is possible to gradually charge the submodule capacitor during startupwhich otherwise could draw the very high charging current. This feature points the progressivelysetting up the HVDC link voltage. In summary, this model allows to analyze the performance of MMCbased multiterminal HVDC network during normal and fault condition using relatively inexpensivecomputational device without leaving most of its dynamics.


RTDS is a high computing device and works as real time simulator which facilitates the interactionwith the physical hardware. It uses only fixed step discrete solver, whereas non-real time variablestep solver and continuous model could be used in MATLAB Simulink. The sample time for powerelectronics switch is 2.3 µs and for controller is 50 µs. We have used a multirack RTDS platform tosimulate such a large system considering detailed MMC model in real time. PB5 and Nova core formthe multirack RTDS hardware platform. MMC1, MMC2, and MMC3 are placed in one rack, and MMC4with wind farm are placed in another rack.

One wind generator of 2 MW with its optimum controller is developed and then scaled to 100 MWby imitating the behavior of one unit. The runtime RTDS RSCAD interface is presented in Figure 11with running data. Tables 1 and 2 provide the detailed parameters for the complete system.



In this way, the lower or upper arm MMC of each phase shows the capacitance voltage that is governed by the Equation (23). Unlike average model, aggregate model also facilitates the blocking option of MMC during simulation. Current flow from point A to point B could be stopped, if the Nblock

is set equal to N. Similarly, the combination of Nmodule and Nblock yields the current flow from zero to maximum. In addition, it is possible to gradually charge the submodule capacitor during startup which otherwise could draw the very high charging current. This feature points the progressively setting up the HVDC link voltage. In summary, this model allows to analyze the performance of MMC based multiterminal HVDC network during normal and fault condition using relatively inexpensive computational device without leaving most of its dynamics.


RTDS is a high computing device and works as real time simulator which facilitates the interaction with the physical hardware. It uses only fixed step discrete solver, whereas non-real time variable step solver and continuous model could be used in MATLAB Simulink. The sample time for power electronics switch is 2.3 µs and for controller is 50 µs. We have used a multirack RTDS platform to simulate such a large system considering detailed MMC model in real time. PB5 and Nova core form the multirack RTDS hardware platform. MMC1, MMC2, and MMC3 are placed in one rack, and MMC4 with wind farm are placed in another rack.

One wind generator of 2 MW with its optimum controller is developed and then scaled to 100 MW by imitating the behavior of one unit. The runtime RTDS RSCAD interface is presented in Figure 11 with running data. Table 1 and 2 provide the detailed parameters for the complete system.

Figure 11. RTDS run time interface of the complete system.

Vin

Vblock

IinIblock

A

B

Figure 11. RTDS run time interface of the complete system with running data.


Table 1. Wind turbine and converter data.

Quantity Value

Wind turbine parametersRated turbine power 2 MW

Generator speed at rated turbine speed 1 puRated wind speed 12 m/s

Squirrel cage induction generator and controllerparameters

Nominal power 2 MWStator voltage (L-L) 690 V

Rated frequency 50 HzStator resistance, Rs 1 mΩRotor resistance, Rr 1.3 mΩ

Total stator inductance, Ls 2.55 mHTotal rotor inductance, Lr 2.56 mH

Magnetizing inductance, Lm 2.44 mHPI1 311 + 1400/s puPI11 180 + 112.5/s pu

Grid side voltage source converter parametersDC link Voltage 1.5 kV

Transformer rated power 2.2 MVATotal inductance 0.20 puTotal resistance 0.005 pu

PI3 0.64 + 5/s puPI4 1 + 100/s pu

Table 2. MMC controller data.

Quantity Value

MMC Controller parametersRated voltage (L-L) 100 kV

Rated power 500 MWRated frequency 50 Hz

Rated DC link voltage 200 kVTotal number of submodule per arm 200

Arm inductance 0.15 puArm resistance 0.0015 pu

Modulation Nearest levelMMC 4 AC voltage controller parameters

PI7 (0.6 + 6/s) puMMC DC link voltage controller parameters

PI4 (4 + 100/s) puMMC P and Q controller parameters

PI5 = PI6 (0.17 + 1/s) puALL MMC current controller parameters

PI8 (0.6 + 6/s) puModulation Nearest level

Short circuit ratio of strong AC grids 10AC grids X/R ratio 7DC line resistance 1.39 mΩ/km

DC line inductance 0.159 mH/kmDC line capacitance 0.231 µF/km

3.1. HVDC Link Voltage Control and Optimum Wind Energy Integration

Figure 12 shows the HVDC link voltage and AC voltage controller performance. MMC1 is keptblocked until 1.5 s and then unblocked to control the HVDC link voltage gradually towards 200 kV.During the blocking period, the current limiting resistor is active and then bypassed at 1.5 s. DC voltage


reaches its reference around 3 s. After HVDC link voltage settles, MMC4 starts controlling the ACvoltage to provide the support for renewable energy integration.



Figure 12 shows the HVDC link voltage and AC voltage controller performance. MMC1 is kept

blocked until 1.5 s and then unblocked to control the HVDC link voltage gradually towards 200 kV.

During the blocking period, the current limiting resistor is active and then bypassed at 1.5s. DC

voltage reaches its reference around 3 s. After HVDC link voltage settles, MMC4 starts controlling

the AC voltage to provide the support for renewable energy integration.

Figure 12. MMC1 HVDC link voltage and MMC4 AC side voltage controller.

As can be seen from Figure 12, AC voltage settles within 1s. Actual Vd and Vq follow the

commanded reference signal. Vq reference was set to zero for a balanced set of three-phase voltage.

Figure 13 and Figure 14 show a similar result in RTDS. RTDS generates large number of data in real

time which points inability to record longer simulation. Therefore, shorter window is used to capture

the simulation where starting time zero means any running time. RTDS results from Figure 15

validates the controller performance for optimum wind energy tracking in real time during wind

speed changes from 6 ms-1 to 12 ms-1. From Figure 2, the optimum wind power is 0.2 MW at 6 ms-1,

whereas 2 MW at 12 ms-1. It is inferred from Figure 15 that 50 units of 2 MW wind generators deliver

power 10 MW (0.2 × 50 = 10 MW) to 100MW (2 × 50 = 100 MW) during such wind speed change

through MMC4 to HVDC transmission network. The machine side VSC current controller tracks the

reference current with small overshoot and almost zero steady state error as shown in Figure 16 over

such wind speed change. The magnetizing current IDS_Ref of machine side VSC remains constant

and torque controlling current IQS_Ref changes with reference torque.

Figure 13. MMC1-HVDC link voltage control in RTDS.

0 0.71608 1.43216 2.14824 2.86433 3.58041 4.29649

0.341

0.466

0.591

0.716

0.841

0.966

1.091

DC lin

k volt

age

(pu)

VDCpu_Ref VDCpu_actual


As can be seen from Figure 12, AC voltage settles within 1 s. Actual Vd and Vq follow thecommanded reference signal. Vq reference was set to zero for a balanced set of three-phase voltage.Figures 13 and 14 show a similar result in RTDS. RTDS generates large number of data in real timewhich points inability to record longer simulation. Therefore, shorter window is used to capture thesimulation where starting time zero means any running time. RTDS results from Figure 15 validatesthe controller performance for optimum wind energy tracking in real time during wind speed changesfrom 6 ms−1 to 12 ms−1. From Figure 2, the optimum wind power is 0.2 MW at 6 ms−1, whereas2 MW at 12 ms−1. It is inferred from Figure 15 that 50 units of 2 MW wind generators deliver power10 MW (0.2× 50 = 10 MW) to 100 MW (2× 50 = 100 MW) during such wind speed change throughMMC4 to HVDC transmission network. The machine side VSC current controller tracks the referencecurrent with small overshoot and almost zero steady state error as shown in Figure 16 over such windspeed change. The magnetizing current IDS_Ref of machine side VSC remains constant and torquecontrolling current IQS_Ref changes with reference torque.



Figure 12 shows the HVDC link voltage and AC voltage controller performance. MMC1 is kept blocked until 1.5 s and then unblocked to control the HVDC link voltage gradually towards 200 kV. During the blocking period, the current limiting resistor is active and then bypassed at 1.5s. DC voltage reaches its reference around 3 s. After HVDC link voltage settles, MMC4 starts controlling the AC voltage to provide the support for renewable energy integration.


As can be seen from Figure 12, AC voltage settles within 1s. Actual Vd and Vq follow the commanded reference signal. Vq reference was set to zero for a balanced set of three-phase voltage. Figure 13 and Figure 14 show a similar result in RTDS. RTDS generates large number of data in real time which points inability to record longer simulation. Therefore, shorter window is used to capture the simulation where starting time zero means any running time. RTDS results from Figure 15 validates the controller performance for optimum wind energy tracking in real time during wind speed changes from 6 ms-1 to 12 ms-1. From Figure 2, the optimum wind power is 0.2 MW at 6 ms-1, whereas 2 MW at 12 ms-1. It is inferred from Figure 15 that 50 units of 2 MW wind generators deliver power 10 MW (0.2 × 50 = 10 MW) to 100MW (2 × 50 = 100 MW) during such wind speed change through MMC4 to HVDC transmission network. The machine side VSC current controller tracks the reference current with small overshoot and almost zero steady state error as shown in Figure 16 over such wind speed change. The magnetizing current IDS_Ref of machine side VSC remains constant and torque controlling current IQS_Ref changes with reference torque.


0 0.71608 1.43216 2.14824 2.86433 3.58041 4.296490.341

0.466

0.591

0.716

0.841

0.966

1.091

DC lin

k volt

age (

pu)

VDCpu_Ref VDCpu_actual



Figure 14. MMC4 AC link voltage control in RTDS.

Figure 15. Optimum wind energy integration into MMC4 terminal.

2.98921 3.49101 3.99281 4.4946 4.9964 5.4982 6-0.04

0.15

0.34

0.53

0.72

0.91

1.1 V

olta

ge(p

u)VDC VACd_actual VACd_ref VACq_actual

1.3 2.8 4.2 5.7 7.1 8.6 100

2.1

4.2

6.3

8.3

10.4

12.5

Gen

erat

or a

nd w

ind

spee

d

Generator speed (pu) wind speed m/s-120

-100

-80

-60

-40

-20

0

Real

pow

er e

xcha

nge(

MW

)

MMC1 MMC2 MMC3 MMC4190

193.33

196.67

200

203.33

206.67

DC lin

k vo

ltage

(kV)





2.98921 3.49101 3.99281 4.4946 4.9964 5.4982 6-0.04

0.15

0.34

0.53

0.72

0.91

1.1 V

olta

ge(p

u)VDC VACd_actual VACd_ref VACq_actual

1.3 2.8 4.2 5.7 7.1 8.6 100

2.1

4.2

6.3

8.3

10.4

12.5

Gen

erat

or a

nd w

ind

spee

d

Generator speed (pu) wind speed m/s-120

-100

-80

-60

-40

-20

0

Real

pow

er e

xcha

nge(

MW

)


193.33

196.67

200

203.33

206.67

DC lin

k vo

ltage

(kV)



Figure 16. Performance of current (A) controller of machine side VSC in RTDS.

3.2. Multiterminal Operation of MMC-HVDC Network

Multiterminal operation has been performed in aggregate model based HVDC network in MATLAB Simulink and detail model based HVDC network in RTDS. As can be seen in Figure 17, MMC2 starts receiving energy from AC grids 2 at 5 s, whereas MMC3 starts injecting energy into AC grids 3 at 7 s.

Twenty-five units of 2 MW wind generators start delivering power through MMC4 to HVDC network. MMC1 controls the DC link voltage and balances the energy mismatch in the multiterminal HVDC network. Negligible DC link voltage overshoot has been produced during energy exchange.

Figure 18 demonstrates the multiterminal power exchange in the HVDC network in RTDS. MMC2 delivers 35 MW power to the AC grids 2 and MMC3 injects 60 MW power to the HVDC link while MMC4 receives 100 MW wind energy. Small overshoot in the HVDC link voltage has been observed in RTDS. MMC1 transfers around 125 MW energy from the HVDC link to AC grids 1 in the steady state. Any excess or shortage of energy increases or decreases the DC link voltage, which produces error signal to the HVDC link voltage regulator. The controller adjusts the real power reference current to keep the DC link voltage regulated. Unlike AC transmission network, no reactive power flow across the HVDC transmission line. However, converter could exchange reactive power with the AC network. Figure 11 shows reactive power exchange of 10 MVar, 10 MVar and 15 MVar with AC grids 1, 2 and 3 respectively. Besides, converter provides the reactive power support during fault in the AC network.

0 1.66667 3.33333 5 6.66667 8.33333 10-2,000

-1,333.333

-666.667

0

666.667

1,333.333

2,000IDS_Ref IDS_A IQS_Ref IQS_A

Figure 16. Performance of current (A) controller of machine side VSC in RTDS.

3.2. Multiterminal Operation of MMC-HVDC Network

Multiterminal operation has been performed in aggregate model based HVDC network in MATLABSimulink and detail model based HVDC network in RTDS. As can be seen in Figure 17, MMC2 startsreceiving energy from AC grids 2 at 5 s, whereas MMC3 starts injecting energy into AC grids 3 at 7 s.

Twenty-five units of 2 MW wind generators start delivering power through MMC4 to HVDCnetwork. MMC1 controls the DC link voltage and balances the energy mismatch in the multiterminalHVDC network. Negligible DC link voltage overshoot has been produced during energy exchange.

Figure 18 demonstrates the multiterminal power exchange in the HVDC network in RTDS. MMC2delivers 35 MW power to the AC grids 2 and MMC3 injects 60 MW power to the HVDC link whileMMC4 receives 100 MW wind energy. Small overshoot in the HVDC link voltage has been observed inRTDS. MMC1 transfers around 125 MW energy from the HVDC link to AC grids 1 in the steady state.Any excess or shortage of energy increases or decreases the DC link voltage, which produces errorsignal to the HVDC link voltage regulator. The controller adjusts the real power reference current tokeep the DC link voltage regulated. Unlike AC transmission network, no reactive power flow acrossthe HVDC transmission line. However, converter could exchange reactive power with the AC network.Figure 11 shows reactive power exchange of 10 MVar, 10 MVar and 15 MVar with AC grids 1, 2 and 3respectively. Besides, converter provides the reactive power support during fault in the AC network.


Figure 17. Energy exchange in MMC based multiterminal HVDC transmission network in MATLAB Simulink

Figure 18. Power exchange in multiterminal HVDC network in RTDS.

3.3. Balanced and Unbalanced Fault Analysis

Far away from being perfectly constant, balanced and stable, the behavior of the electrical

network is influenced by different kinds of balanced and unbalanced faults. Therefore, the control of

grid-connected power converters must guarantee a proper performance under such operating

conditions. Usually, the grid connected converter needs to remain connected for a minimum duration

(150 ms) during faults and inject reactive power. As the MMC1 works as the master controller and

2.07 2.75 3.44 4.12 4.8 5.48 6.16

-130

-95

-60

-25

10

45

80

Real

powe

r exc

hang

e(M

W)

MMC1 MMC2 MMC3 MMC4

190

193.33

196.67

200

203.33

206.67

210

DC lin

k volt

age

(kV)

MMC DC link voltage

Figure 17. Energy exchange in MMC based multiterminal HVDC transmission network inMATLAB Simulink.


Figure 17. Energy exchange in MMC based multiterminal HVDC transmission network in MATLAB Simulink



Far away from being perfectly constant, balanced and stable, the behavior of the electrical network is influenced by different kinds of balanced and unbalanced faults. Therefore, the control of grid-connected power converters must guarantee a proper performance under such operating conditions. Usually, the grid connected converter needs to remain connected for a minimum duration (150 ms) during faults and inject reactive power. As the MMC1 works as the master controller and

2.07 2.75 3.44 4.12 4.8 5.48 6.16-130

-95

-60

-25

10

45

80

Real

powe

r exc

hang

e(MW

)


193.33

196.67

200

203.33

DC lin

k volt

age (

kV)



Far away from being perfectly constant, balanced and stable, the behavior of the electricalnetwork is influenced by different kinds of balanced and unbalanced faults. Therefore, the controlof grid-connected power converters must guarantee a proper performance under such operatingconditions. Usually, the grid connected converter needs to remain connected for a minimum duration


(150 ms) during faults and inject reactive power. As the MMC1 works as the master controller andremaining MMCs operate as slave controller, fault analysis has been carried out only on the MMC1connected AC grids to evaluate the performance of HVDC link voltage and reactive power controllerwith grid code compliance. During different kinds of faults, MMC4 received 100 MW wind energy,whereas the power flow of MMC2 and MMC3 was kept zero. It is also noted that fault analysis hasbeen carried out for the aggregate model based MMC network in MATLAB Simulink and detailedmodel based MMC network in RTDS.

3.3.1. AC Grids Frequency Change at PCC1

While MMC1 is transferring 100 MW wind power to AC grids 1, frequency is changed from 50 Hzto 48 Hz at 12 s and 50 Hz to 52 Hz at 14 s at PCC1 as shown in Figure 19 to evaluate the performance ofthe HVDC link voltage controller. Due to sudden PCC1 frequency change, small overshoot is observedin both HVDC link voltage and power exchange. Figure 20 shows the performance of a detailed modelof MMC in RTDS for such change. The result from RTDS shows almost similar performance with theoutcome from MATLAB Simulink. The quick response of PLL in retrieving the phase and frequencyinformation of the AC grid and decoupled current controller counteract the frequency change of theAC grids. However, small overshoot is observed during transient but remain constant and regulatedin the steady state.


remaining MMCs operate as slave controller, fault analysis has been carried out only on the MMC1

connected AC grids to evaluate the performance of HVDC link voltage and reactive power controller

with grid code compliance. During different kinds of faults, MMC4 received 100 MW wind energy,

whereas the power flow of MMC2 and MMC3 was kept zero. It is also noted that fault analysis has

been carried out for the aggregate model based MMC network in MATLAB Simulink and detailed

model based MMC network in RTDS.

3.3.1. AC Grids Frequency Change at PCC1

While MMC1 is transferring 100 MW wind power to AC grids 1, frequency is changed from 50

Hz to 48 Hz at 12 s and 50 Hz to 52 Hz at 14 s at PCC1 as shown in Figure 19 to evaluate the

performance of the HVDC link voltage controller. Due to sudden PCC1 frequency change, small

overshoot is observed in both HVDC link voltage and power exchange. Figure 20 shows the

performance of a detailed model of MMC in RTDS for such change. The result from RTDS shows

almost similar performance with the outcome from MATLAB Simulink. The quick response of PLL

in retrieving the phase and frequency information of the AC grid and decoupled current controller

counteract the frequency change of the AC grids. However, small overshoot is observed during

transient but remain constant and regulated in the steady state.

Figure 19. Performance of HVDC link voltage controller of MMC1 during over/under frequency of

PCC1 voltage in MATLAB Simulink.

Figure 19. Performance of HVDC link voltage controller of MMC1 during over/under frequency ofPCC1 voltage in MATLAB Simulink.


Figure 20. Performance of DC link voltage controller during over/under frequency of PCC1 voltage in RTDS.

3.3.2. SLG, DLG and 3LG Fault at PCC1

Three type 500 ms duration faults (SLG, DLG, 3LG) have been applied at the PCC1 terminal to evaluate the controller performance while MMC1 is transferring 100 MW wind power to PCC1 terminal. Aggregate and detailed model based MMC-HVDC network have been used in MATLAB Simulink and RTDS respectively for fault analysis. Since the fault limits the energy transfer capability of the converter, the HVDC link voltage could go up if the excess energy is not controlled. In this work, a dynamic braking resistor (DBR, a switch with a series-connected resistor (500 Ω)) is employed to handle excess energy in the HVDC link. If the HVDC link voltage exceeds 1.2pu, it activates the DBR and deactivates when HVDC link voltage goes below 1.1 pu. Besides real power mismatch control, it is expected to inject reactive power to improve the PCC1 voltage. This work has prioritized the HVDC link voltage controller over the reactive power controller. Figure 21 to Figure 24 show the power exchange and HVDC link voltage controller during SLG, DLG and 3LG faults in RTDS and MATLAB Simulink respectively. As the converter power capacity is 200 MW (1pu), the converter 0.5pu power transfer has not been affected except the start of fault occurrence, which has increased the HVDC link voltage below the threshold of DBR activation voltage during SLG. During this period, average 0.45 per unit reactive power has been injected. However, power exchange encounters oscillation at twice-fundamental frequency due to negative sequence voltage. During DLG, real power transfer is limited to average 0.25 pu, and reactive power is around 0.25 pu. It is also seen from Figure 22 that DBR dissipated excess energy and controlled HVDC link voltage within its range. Oscillation at a twice-fundamental frequency in the power transfer is also observed during DLG. During 3LG fault in Figure 23, real power transfer remains zero and maximum reactive power is injected at fault clearing time. During this fault, DBR fully dissipated the excess energy and remain activated throughout the fault period to control the HVDC link voltage. The results from the aggregate model based MMC in MATLAB Simulink, as shown in Figure 24, provides an almost

1.24208 1.78507 2.32805 2.87104 3.41403 3.95701 4.50

0.20.40.60.8

11.2

Change duration47484950515253

Frequency (Hz)-105

-103.33-101.67

-100-98.33-96.67

-95Power exchange (MW)

0.970.980.99

11.011.021.04

MMC DC Link (pu)

Figure 20. Performance of DC link voltage controller of MMC1 during over/under frequency of PCC1voltage in RTDS.

3.3.2. SLG, DLG and 3LG Fault at PCC1

Three type 500 ms duration faults (SLG, DLG, 3LG) have been applied at the PCC1 terminalto evaluate the controller performance while MMC1 is transferring 100 MW wind power to PCC1terminal. Aggregate and detailed model based MMC-HVDC network have been used in MATLABSimulink and RTDS respectively for fault analysis. Since the fault limits the energy transfer capabilityof the converter, the HVDC link voltage could go up if the excess energy is not controlled. In this work,a dynamic braking resistor (DBR, a switch with a series-connected resistor (500 Ω)) is employed tohandle excess energy in the HVDC link. If the HVDC link voltage exceeds 1.2 pu, it activates the DBRand deactivates when HVDC link voltage goes below 1.1 pu. Besides real power mismatch control, it isexpected to inject reactive power to improve the PCC1 voltage. This work has prioritized the HVDClink voltage controller over the reactive power controller. Figures 21–24 show the power exchangeand HVDC link voltage controller during SLG, DLG and 3LG faults in RTDS and MATLAB Simulinkrespectively. As the converter power capacity is 200 MW (1 pu), the converter 0.5 pu power transfer hasnot been affected except the start of fault occurrence, which has increased the HVDC link voltage belowthe threshold of DBR activation voltage during SLG. During this period, average 0.45 per unit reactivepower has been injected. However, power exchange encounters oscillation at twice-fundamentalfrequency due to negative sequence voltage. During DLG, real power transfer is limited to average0.25 pu, and reactive power is around 0.25 pu. It is also seen from Figure 22 that DBR dissipatedexcess energy and controlled HVDC link voltage within its range. Oscillation at a twice-fundamentalfrequency in the power transfer is also observed during DLG. During 3LG fault in Figure 23, real powertransfer remains zero and maximum reactive power is injected at fault clearing time. During this fault,DBR fully dissipated the excess energy and remain activated throughout the fault period to controlthe HVDC link voltage. The results from the aggregate model based MMC in MATLAB Simulink,as shown in Figure 24, provides an almost similar outcome comparing to RTDS provided Figures 21–23.


In all cases, HVDC link voltage did not exceed 1.2 pu because of DBR. After fault clearance, the powerflow becomes normal within 0.062 s for SLG fault, 0.14 s for DLG fault, and 0.167 s for 3LG fault inRTDS as depicted in Figures 21–23. MATLAB Simulink result as shown in Figure 24 also shows theincreasing settling time from SLG to 3LG fault. HVDC link voltage controller takes shorter settlingtime after SLG fault clearance. The recovery time of the HVDC link voltage controller after 3LG faultclearance is longer than other types of fault. The MATLAB Simulink result agrees with the RTDS result.


similar outcome comparing to RTDS provided Figure 21 to Figure 23. In all cases, HVDC link voltage did not exceed 1.2 pu because of DBR. After fault clearance, the power flow becomes normal within 0.062 s for SLG fault, 0.14 s for DLG fault, and 0.167 s for 3LG fault in RTDS as depicted in Figure 21 to Figure 23. MATLAB Simulink result as shown in Figure 24 also shows the increasing settling time from SLG to 3LG fault. HVDC link voltage controller takes shorter settling time after SLG fault clearance. The recovery time of the HVDC link voltage controller after 3LG fault clearance is longer than other types of fault. The MATLAB Simulink result agrees with the RTDS result.

Figure 21. Power exchange and DC link voltage of MMC1 during SLG fault at PCC1 in RTDS.

Figure 22. Power exchange and DC link voltage of MMC1 during DLG fault at PCC1 in RTDS.

0.38549 0.48977 0.59406 0.69835 0.80263 0.90692 1.01120.84

0.9

0.95

1.01

1.06

1.12

1.17MMC VDC link (pu)

-1

-0.5

0

0.5

1

1.5Q_exchange(pu)

-2

-1

0

1P_exchange (pu)

0.3271 0.4634 0.59969 0.73598 0.87227 1.00857 1.144860.8

0.92

1.03

1.15

1.27

1.38


-0.5

0

0.5

1

1.5Q_exchange(pu)

-1.5

-1

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0

0.5P_exchange (pu)



similar outcome comparing to RTDS provided Figure 21 to Figure 23. In all cases, HVDC link voltage did not exceed 1.2 pu because of DBR. After fault clearance, the power flow becomes normal within 0.062 s for SLG fault, 0.14 s for DLG fault, and 0.167 s for 3LG fault in RTDS as depicted in Figure 21 to Figure 23. MATLAB Simulink result as shown in Figure 24 also shows the increasing settling time from SLG to 3LG fault. HVDC link voltage controller takes shorter settling time after SLG fault clearance. The recovery time of the HVDC link voltage controller after 3LG fault clearance is longer than other types of fault. The MATLAB Simulink result agrees with the RTDS result.



0.38549 0.48977 0.59406 0.69835 0.80263 0.90692 1.01120.84

0.9

0.95

1.01

1.06

1.12


-1

-0.5

0

0.5

1

1.5Q_exchange(pu)

-2

-1

0

1P_exchange (pu)

0.3271 0.4634 0.59969 0.73598 0.87227 1.00857 1.144860.8

0.92

1.03

1.15

1.27

1.38


-0.5

0

0.5

1

1.5Q_exchange(pu)

-1.5

-1

-0.5

0

0.5P_exchange (pu)



Figure 23. Power exchange and DC link voltage of MMC1 during 3LG fault at PCC1 in RTDS.

Figure 24. Power exchange and DC link voltage of MMC1 during SLG, DLG and 3LG fault at PCC1 in MATLAB Simulink.

0.3271 0.4891 0.65109 0.81308 0.97508 1.13707 1.299070.8

0.92

1.03

1.15

1.27

1.38


-0.5

0

0.5

1

1.5Q_exchange(pu)

-2

-1

0

1P_exchange (pu)




Figure 24. Power exchange and DC link voltage of MMC1 during SLG, DLG and 3LG fault at PCC1

in MATLAB Simulink.

0.3271 0.4891 0.65109 0.81308 0.97508 1.13707 1.29907

0.8

0.92

1.03

1.15

1.27

1.38

1.5

MMC VDC link (pu)

-0.5

0

0.5

1

1.5

Q_exchange(pu)

-2

-1

0

1

P_exchange (pu)

Figure 24. Power exchange and DC link voltage of MMC1 during SLG, DLG and 3LG fault at PCC1 inMATLAB Simulink.

Figures 25–28 show the performance of Id and Iq current controller in RTDS and MATLAB. VSCis a current limited device, and it might fail if overcurrent is allowed for longer duration. However,


limited overload is allowed during the fault condition. In this work, it is considered maximum 1.1 puand 0.8 pu for Id and Iq current, respectively. As can be seen from Figure 27, the actual current followsthe reference current during balanced fault at PCC1. During an unbalanced fault as depicted inFigures 25 and 26, the average of actual current follows the reference current. Due to the negativesequence voltage, actual current oscillates at a twice-fundamental frequency around the referencecurrent during unbalanced SLG and DLG faults, but no oscillation is observed during balanced 3LGfault. Figure 28 illustrates the performance of current controller in MATLAB Simulink during balancedand unbalanced faults. Both MATLAB Simulink and RTDS simulation yield almost similar results.


Figure 25. Id and Iq current (pu) of MMC1 during SLG at PCC1 in RTDS.

Figure 26. Id and Iq current (pu) of MMC1 during DLG at PCC1 in RTDS.

0.38018 0.48939 0.5986 0.70781 0.81702 0.92623 1.03544-1

0

1

2IQ_R IQ_A

-2

-1

0

1ID_R ID_A

0.38018 0.48939 0.5986 0.70781 0.81702 0.92623 1.03544-0.5

0

0.5

1

1.5IQ_R IQ_A

-2

-1

0

1ID_R ID_A





0.38018 0.48939 0.5986 0.70781 0.81702 0.92623 1.03544-1

0

1

2IQ_R IQ_A

-2

-1

0

1ID_R ID_A

0.38018 0.48939 0.5986 0.70781 0.81702 0.92623 1.03544-0.5

0

0.5

1

1.5IQ_R IQ_A

-2

-1

0

1ID_R ID_A



Figure 27. Id and Iq current of MMC1 during 3LG at PCC1 in RTDS.

Figure 25 to Figure 28 show the performance of Id and Iq current controller in RTDS and MATLAB. VSC is a current limited device, and it might fail if overcurrent is allowed for longer duration. However, limited overload is allowed during the fault condition. In this work, it is considered maximum 1.1 pu and 0.8 pu for Id and Iq current, respectively. As can be seen from Figure 27, the actual current follows the reference current during balanced fault at PCC1. During an unbalanced fault as depicted in Figure 25 and Figure 26, the average of actual current follows the reference current. Due to the negative sequence voltage, actual current oscillates at a twice-fundamental frequency around the reference current during unbalanced SLG and DLG faults, but no oscillation is observed during balanced 3LG fault. Figure 28 illustrates the performance of current controller in MATLAB Simulink during balanced and unbalanced faults. Both MATLAB Simulink and RTDS simulation yield almost similar results.

Figure 28. Id and Iq current of MMC1 during SLG, DLG and 3LG fault at PCC1 in MATLAB

Simulink.

Figure 29 to Figure 32 present the Y-Δ side currents waveform and PCC1 terminal voltage waveform in RTDS and MATLAB Simulink. Since zero sequence currents flow into the Y-side transformer, Y-side current is more unsymmetrical than Δ side current during unbalanced SLG and

0.33463 0.47601 0.61738 0.75875 0.90013 1.0415 1.18288-0.5

0

0.5

1

1.5IQ_R IQ_A

-2

-1

0

1ID_R ID_A

Figure 27. Id and Iq current (pu) of MMC1 during 3LG at PCC1 in RTDS.


Figure 27. Id and Iq current (pu) of MMC1 during 3LG at PCC1 in RTDS.

Figure 25 to Figure 28 show the performance of Id and Iq current controller in RTDS and

MATLAB. VSC is a current limited device, and it might fail if overcurrent is allowed for longer

duration. However, limited overload is allowed during the fault condition. In this work, it is

considered maximum 1.1 pu and 0.8 pu for Id and Iq current, respectively. As can be seen from Figure

27, the actual current follows the reference current during balanced fault at PCC1. During an

unbalanced fault as depicted in Figure 25 and Figure 26, the average of actual current follows the

reference current. Due to the negative sequence voltage, actual current oscillates at a twice-

fundamental frequency around the reference current during unbalanced SLG and DLG faults, but no

oscillation is observed during balanced 3LG fault. Figure 28 illustrates the performance of current

controller in MATLAB Simulink during balanced and unbalanced faults. Both MATLAB Simulink

and RTDS simulation yield almost similar results.

Figure 28. Id and Iq current (pu) of MMC1 during SLG, DLG and 3LG fault at PCC1 in MATLAB

Simulink.

Figure 29 to Figure 32 present the Y-Δ side currents waveform and PCC1 terminal voltage

waveform in RTDS and MATLAB Simulink. Since zero sequence currents flow into the Y-side

transformer, Y-side current is more unsymmetrical than Δ side current during unbalanced SLG and

0.33463 0.47601 0.61738 0.75875 0.90013 1.0415 1.18288

-0.5

0

0.5

1

1.5

IQ_R IQ_A

-2

-1

0

1

ID_R ID_A

Figure 28. Id and Iq current (pu) of MMC1 during SLG, DLG and 3LG fault at PCC1 in MATLAB Simulink.

Figures 29–32 present the Y-∆ side currents waveform and PCC1 terminal voltage waveformin RTDS and MATLAB Simulink. Since zero sequence currents flow into the Y-side transformer,Y-side current is more unsymmetrical than ∆ side current during unbalanced SLG and DLG. However,both side current wave shapes are similar during the balanced three-phase fault. It is also observedthat the maximum phase current remained around 1.36 pu in the ∆ side transformer for all cases.However, the Y-∆ side currents for all phases remain exactly within its permissible limit duringbalanced fault. During unbalanced fault, one phase current in the ∆-side transformer slightly exceedspeak value. Y-side currents are around 2 pu for SLG fault and 2.667 pu for DLG fault. Since DLG faultproduces more negative sequence voltage and significantly limits the power transfer capability, Y-sidetransformer encounters larger unsymmetrical current comparing to SLG fault.



DLG. However, both side current wave shapes are similar during the balanced three-phase fault. It is also observed that the maximum phase current remained around 1.36 pu in the Δ side transformer for all cases. However, the Y-Δ side currents for all phases remain exactly within its permissible limit during balanced fault. During unbalanced fault, one phase current in the Δ-side transformer slightly exceeds peak value. Y-side currents are around 2 pu for SLG fault and 2.667 pu for DLG fault. Since DLG fault produces more negative sequence voltage and significantly limits the power transfer capability, Y-side transformer encounters larger unsymmetrical current comparing to SLG fault.

Figure 29. Grid side voltage (PCC1), Y-side and Δ-side current (pu) of transformer connected MMC1 during SLG fault in RTDS.

Figure 30. Grid side voltage (PCC1), Y-side and Δ-side current (pu) of the transformer connected MMC1 during DLG fault in RTDS.

0.35 0.4891 0.62821 0.76731 0.90641 1.04551 1.18462-2

-1.333

-0.667

0

0.667

1.333

2ISA ISB ISC

-4

-2.847

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0.611

1.764

2.917IPA IPB IPC

-1

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0

0.333

0.667

1VPA VPB VPC

0.32981 0.46106 0.59231 0.72356 0.85481 0.98606 1.11731-2

-1.333

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0

0.667

1.333

2ISA ISB ISC

-4

-2.667

-1.333

0

1.333

2.667

4IPA IPB IPC

-1

-0.667

-0.333

0

0.333

0.667

1VPA VPB VPC

Figure 29. Grid side voltage (PCC1), Y-side and ∆-side current (pu) of transformer connected MMC1during SLG fault in RTDS.


DLG. However, both side current wave shapes are similar during the balanced three-phase fault. It is also observed that the maximum phase current remained around 1.36 pu in the Δ side transformer for all cases. However, the Y-Δ side currents for all phases remain exactly within its permissible limit during balanced fault. During unbalanced fault, one phase current in the Δ-side transformer slightly exceeds peak value. Y-side currents are around 2 pu for SLG fault and 2.667 pu for DLG fault. Since DLG fault produces more negative sequence voltage and significantly limits the power transfer capability, Y-side transformer encounters larger unsymmetrical current comparing to SLG fault.

Figure 29. Grid side voltage (PCC1), Y-side and Δ-side current (pu) of transformer connected MMC1 during SLG fault in RTDS.

Figure 30. Grid side voltage (PCC1), Y-side and Δ-side current (pu) of the transformer connected MMC1 during DLG fault in RTDS.

0.35 0.4891 0.62821 0.76731 0.90641 1.04551 1.1846-2

-1.333

-0.667

0

0.667

1.333

2ISA ISB ISC

-4

-2.847

-1.694

-0.542

0.611

1.764

2.917IPA IPB IPC

-1

-0.667

-0.333

0

0.333

0.667

1VPA VPB VPC

0.32981 0.46106 0.59231 0.72356 0.85481 0.98606 1.11731-2

-1.333

-0.667

0

0.667

1.333

2ISA ISB ISC

-4

-2.667

-1.333

0

1.333

2.667

4IPA IPB IPC

-1

-0.667

-0.333

0

0.333

0.667

1VPA VPB VPC

Figure 30. Grid side voltage (PCC1), Y-side and ∆-side current (pu) of the transformer connectedMMC1 during DLG fault in RTDS.


Figure 31. Grid side voltage (PCC1), Y-side and Δ-side current (pu) of the transformer connected MMC1 during 3LG fault in RTDS.

Figure 32. Grid side voltage (PCC1), Y-side and Δ-side current of transformer connected MMC1 during SLG, DLG and 3LG fault in MATLAB Simulink.

4. Conclusions

This research develops an MMC based multiterminal HVDC transmission network for optimum wind energy integration. Field-oriented control for optimum tracking of wind energy has been

0.36346 0.48013 0.59679 0.71346 0.83013 0.9468 1.06346-2

-1.333

-0.667

0

0.667

1.333

2ISA ISB ISC

-3

-2.028

-1.056

-0.083

0.889

1.861

2.833IPA IPB IPC

-1

-0.667

-0.333

0

0.333

0.667

1VPA VPB VPC

Figure 31. Grid side voltage (PCC1), Y-side and ∆-side current (pu) of the transformer connectedMMC1 during 3LG fault in RTDS.


Figure 31. Grid side voltage (PCC1), Y-side and Δ-side current (pu) of the transformer connected

MMC1 during 3LG fault in RTDS.

Figure 32. Grid side voltage (PCC1), Y-side and Δ-side current of transformer connected MMC1

during SLG, DLG and 3LG fault in MATLAB Simulink.

4. Conclusions

This research develops an MMC based multiterminal HVDC transmission network for optimum

wind energy integration. Field-oriented control for optimum tracking of wind energy has been

0.36346 0.48013 0.59679 0.71346 0.83013 0.9468 1.06346

-2

-1.333

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2

ISA ISB ISC

-3

-2.028

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0.889

1.861

2.833

IPA IPB IPC

-1

-0.667

-0.333

0

0.333

0.667

1

VPA VPB VPC

Figure 32. Grid side voltage (PCC1), Y-side and ∆-side current of transformer connected MMC1 duringSLG, DLG and 3LG fault in MATLAB Simulink.

4. Conclusions

This research develops an MMC based multiterminal HVDC transmission network for optimumwind energy integration. Field-oriented control for optimum tracking of wind energy has beenimplemented. Furthermore, Feed forward controller has been used to form isolated AC network.


It used aggregate model to represent MMC in MATLAB/Simulink and detailed model in RTDS.Both simulation and experimental results are in full agreement. The results confirm the efficacy ofthe proposed MPP controller during wind speed change. The proposed controller performance hasbeen evaluated during symmetrical and unsymmetrical faults at the PCC. The results demonstrate theeffectiveness of the proposed control strategy to enhance the fault ride through capability of the systemwithout violating current and DC link voltage limits.

The main outcomes of this study can be highlighted as follows:

SCIG based optimum wind energy integrated MMC-HVDC network was developed; The dynamics of complete renewable energy farm was considered; Aggregate model based MMC-HVDC network was developed in MATLAB Simulink; Detailed model based MMC-HVDC network was developed in RTDS; The effects of severe balanced and unbalanced faults were analyzed; Control strategy was developed to fulfill the grid code requirements, converter current and HVDC

link voltage limits.

Author Contributions: All authors collaborated on this work. All authors have read and agreed to the publishedversion of the manuscript.

Funding: This research received no external funding.

Acknowledgments: The authors would like to acknowledge the support provided by King Fahd University ofPetroleum & Minerals through the Research Group funded project #DF191004. The authors also acknowledge thefunding support by King Abdullah City for Atomic and Renewable Energy (K.A. CARE), Energy Research &Innovation Center (ERIC) at KFUPM.

Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

Wind Generator and SCIG Side Converter:SCIG Squirrel cage induction generatorMSC Machine side converterRs, Rr SCIG stator and rotor resistanceis SCIG stator current.Vd & Vq SCIG stator d-q axis voltage.id & iq SCIG stator d-q axis current.imr SCIG magnetizing current.ωr Rotor angular speedω0 Angular speed of SCIG stator voltageVline to line rms SCIG stator line to line rms voltageGrid Side Converter (GSC):R, L Reactor resistance and inductance,PCC Point of common coupling,ω0 PCC angular frequency,isd & isq PCC d-q axis current,Vsd & Vsq PCC d-q axis voltage,Vd & Vq VSC terminal d-q axis voltageC DC link capacitance,VDC DC link voltage,Idclink DC link currentMMC Converter:R, L Arm reactor resistance and inductance,PCC1 Point of common coupling of AC grid 1ω0 AC grid angular frequency,Vs, Is AC grid 3-Ø voltage and current,Vsd & Vsq AC grid d-q axis voltage,


isd & isq Ac grid d-q axis current,Vd & Vq MMC terminal d-q axis voltage,VDC HVDC link voltage,Idclink HVDC link currentC Submodule capacitance,Cd DC link pole-to-pole capacitanceN number of submodulesid1 & iq1 Negative sequence d-q axis current,Vd1 & Vq1 Negative sequence d-q axis voltage,SLG Single line to groundDLG Double line to ground3LG Three line to groundDBR Dynamic braking resistor

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