Dynamic performance of the modular seriesconnected converter in a 100 kVdc- transformerless

offshore wind turbine

Sverre GjerdeDep. of Electric Power Engineering

Norwegian University of Science and TechnologyTrondheim, Norway

Email: sverre.gjerde@ntnu.no

Tore M. UndelandDep. of Electric Power Engineering

Norwegian University of Science and TechnologyTrondheim, Norway

Email: tore.undeland@ntnu.no

Abstract—The analysis of a series connected modular convertersuitable for a light weight transformerless offshore wind turbineis presented in this paper. First, an overview of the researchwithin in the field of transformerless turbines is given. Then,the special generator/converter concept is presented, with abrief introduction to the control system principles. Based onsimulations in PSCAD, the impact and benefits of applying aflux weakening scheme in this special application is assessed.Additionally, a possible black-start strategy is demonstrated.Finally, the consequences of an abrupt loss of grid is analysed.The turbine transient response to such a fault is simulated, andthe requirements for fault-mitigation are identified.

I. INTRODUCTION

A. Background

There is assumed to be a significant cost reduction asso-ciated with increased turbine size in offshore wind power[1]. Therefore, both academia and industry are looking intotechnologies which enables 10 MW+ turbines. An example isthe NOWITECH reference turbine [2]. However, an upscalingof today’s turbine technology will result in an increased drivetrain mass. According to [3], the mass of a conventional, 3-phase 10 MW direct drive permanent magnet synchronousgenerator (DD-PMSG) alone will be approximately 300 tons.Therefore, other machine technologies are investigated, suchas the IronLess stator PMSG of the ST10 turbine of Sway [4].

B. Motivation for the transformerlesss option

The standard, low nominal voltage (690 Vrms) introducesanother issue when extrapolating today’s standard solutionsto 10 MW and more. At this voltage rating, the cables fromthe nacelle to ground will be bulky, heavy and stiff. Mediumvoltage solutions are considered [2], but this will not makethe distribution transformer superfluous. Therefore, it has beenproposed to locate the transformer in the top. This adds weightto the already heavy nacelle and results in a more demandingmechanical construction. Equally important is the Operationand Maintenance (O&M) issue: a transformer cannot be mademodular. Hence, a large offshore crane is necessary to performmaintenance if a transformer fails. Such cranes are both scarceand expensive. It is therefore believed to be beneficial bothfor the turbine construction itself and O&M-cost to omit thistransformer.

0

AC

DC

AC

DC

AC

DC

AC

DC

Module 1

Module 9

+11.1 kV

-

+11.1 kV

-

+11.1 kV

-

+100 kV

-

+11.1 kV

-

Generator segment

Converter module

Fig. 1. Proposed modular converter system sketched for N=9 modulesconnected in series to synthesize 100 kVdc output from the light weightturbine.

An additional argument for transformerless power electronicconverters is the introduction of DC-distribution grids [5]. Aspecial transformerless turbine can save DC/DC-conversionstages as well as provide a DC-output directly without adedicated rectifier circuit.

C. Towards light weight, transformerless turbine technology

The IronLess-stator PMSG (IL-PMSG) has been proposed asan alternative to the heavy, conventional direct drive PMSG so-lutions proposed for offshore wind power [4]. A large-diameterIL-PMSG is also the base-technology for a solution whichmitigates issues related to high voltage electric machineryinsulation [6]. If integrated with a suitable power electronicconverter [7], standard winding insulation thickness can beapplied. The result is a high voltage output, without the weightincrease associated with cable wound generators (e.q. [8]).

D. Proposed system

The proposed system is presented in Fig.1. The stator of theironless generator is a modular construction, where each stator

978-1-4799-0002-2/13/$31.00 ©2013 IEEE This is a DRAFT. As such it may not be cited in other works. The citable Proceedings of the Conference will be published in IEEE Xplore shortly after the conclusion of the conference.

TABLE I. SUMMARY, RESEARCH ON TRANSFORMERLESS TURBINE CONCEPTS

System Generator Converter Output voltage Gen.weightConcept 1 Spoked Lightweight Machine CHB1 11 kVrms Light weightConcept 2 PMSG Multi coil CHB2 6-35 kVrms N/AConcept 3 Multi 6 phase PMSG CHB3 11 kVrms N/AConcept 4 Multistar - PMSG Modular VSC 23.6 kVdc N/AConcept 5 PMSG Matrix Design param. StandardConcept 6 Cable -PMSG Diode and VSC ≥ 20 kVrms HighConcept 7 Electrostatic DC N/A 100-300 kV Standard

segment provides a 3-phase output. Each of these 3-phaseoutputs is connected to its own AC/DC-converter module.The DC-buses of these modules are series connected, andhence synthesizing high voltage without the transformer. Con-sequently, operation and maintenance is facilitated, since thesystem is divided in small parts which are easier to handle.

E. Contribution of this paper

This paper presents the continued analysis of a series con-nected converter suitable for a 10 MW, 100 kVdc transformer-less offshore wind turbine presented in [7], [9], [10]. Thework is part of an ongoing PhD-project. The analysis is basedon the converter topology proposed in [11], and builds onthe insulation system [6]. While earlier analysis have focusedon the functionality of the converter chain and the controlsystem, this work focuses on the dynamic performance underconditions outside the nominal operation. A flux weakeningscheme is applied to the converter control to improve thedynamic performance.

A black start method of a turbine with the proposed converterstructure tied to an offshore HVDC-grid is presented. Addi-tionally, the turbine is analysed for loss of grid connection.The analysis is focused on identifying the necessary additionsfor the system to ride through this type of fault, and respectthe technical constraints. Finally, a qualitative discussion onthe system voltage levels is included.

II. OVERVIEW OF RESEARCH ON TRANSFORMELESSOFFSHORE WIND TURBINE CONCEPTS

Several transformerless generator/converter concepts have beenintroduced in order to increase the output voltage from the tur-bine without the distribution transformer. This section presentsan overview of the existing research in this field, with emphasison different converter solutions.

Single phase converter modules: In [12], [13] (concept 1,Tab.I), a special generator is introduced. The generator hasmultiple, concentrated coils, and each of these is connected toa single phase AC/DC-converter, Fig.2(a). The rectified DC-voltage is inverted using H-bridge modules. An 11 kVac outputis synthesized by cascading the H-bridge inverters on theoutput. A similar converter concept is proposed in [14], [15],[16](concept 2, Fig.2(b)). In [17] (concept 3, Fig.3 ), a systemwith several parallel, six phase permanent magnet synchronousgenerators mounted on the same shaft is introduced. The multi-machine approach allows for multiple stator outputs withoutspecial stator core designs. The converter solution is similar toconcept 1 and 2.

(a)

(b)

Fig. 2. Single phase converter solutions for transformerless wind turbineconverter concept a) Conecept 1 b) Concept 2

Fig. 3. Multi generator converter solution for transformerless wind turbineconcept 3.

Fig. 4. Converter solution for multiple, electrically isolated 3-phase statorwinding groups, Concept 4.

Fig. 5. Topology of PMSG+matrix converter for series connection of offshorewind turbines.

Three phase converter modules: A different concept, builtaround 3-phase converter modules is introduced in [11] (con-cept 4). This system consists of four, three-level neutral pointclamped (NPC) converters connected to a generator with fourelectrically isolated stator windings. The DC-buses of theconverter modules are connected in series to synthesize dis-tribution level voltage, 23.6 kVdc. A similar converter solutionis patented in [18].

Series connected wind turbines: [19], [20] (concept 5) presenta solution where each turbine operates with a medium voltageoutput, and the turbines are connected in series to achieve asuitable transmission voltage. The concept employs a standardPMSG with a reduced matrix converter and a high frequencytransformer. Therefore, the concept cannot be said to be trulytransformerless. However, the high frequency transformer isboth light weight and compact compared to the standard 50Hz transformer.

Other concepts: The concepts summarized so far depend ona special converter configurations to synthesize a distributionvoltage. However, there exists also a few other solutions forachieving a high voltage output. One is to use cables forthe stator windings. The insulation of the system voltage isincluded directly in the stator, thus making it possible toachieve a high voltage directly from the 3-phase output of thegenerator [8] (concept 6). For such a system, a high voltage,three phase multilevel converter should be used for the gridinterface. The application of cable windings results in highinsulation thickness, and hence large slots and finally low fillfactor. The PowerFormer is therefore most suitable in specialapplications where space, and not weight, is the limiting factor.

A completely different approach is presented in [21]: an

+Udc,i

-

Module i

(a)

ωestLs

ωestLS

iq,ref,i

ψ

uds,ref

uqs,ref

udc,ref

udc,i

iq,bal,i

ωest

++

-

-

+

-

-

Flux estimator PLL

ψdid,iq

Uds,ref, Uqs,ref ψq

id

iq

id,ref

me,ref

θest

(b)

Fig. 6. Details for a module in the modular series connected convertertopology a) Topology of a single module converter. b) Converter modulecontrol system with vector current control, sensorless rotor position trackingand DC-bus voltage control.

electrostatic, variable-capacitance generator is proposed, witha claimed potential of 100-300 kVdc (Concept 7).

Summary of the concepts for transformerless wind turbines:The overview of the existing research on transformer-lesssolutions for wind power reveals one thing. There is no realconsensus on how to approach the problem: the single phasesolutions have received most attention so far, most likelybecause these fit well with the cascaded H-bridge invertertopologies. However, 3-phase converters have lower compo-nent count and bus capacitance demand for a given voltageand power rating. Series connection of three-phase VoltageSource Converter-bridges (VSC) is not much explored in theliterature.

A series connection of turbines posses its own issues. Mostdominant is the challenging coordinated control of turbines,the fact that all turbines have to be in series in most concepts,and insulation of the turbines.

III. SYSTEM DESCRIPTION

A. Topology

The special generator in this system incorporates an insulationtechnology [6] which allows high machine voltage outputwithout cable windings. Hence, a higher fill factor can beachieved in the machine, and consequently a comparablylighter construction. A prerequisite for the insulation systemis the division of the output voltage into N levels, where N isthe number of isolated stator 3-phase windings (segments). Ithas earlier been stated that N=9 results in an acceptable trade-off between stator weight and system complexity for a 100kVdc output. Hence, the discussions in this work is based on9 modules. The insulation around each segment handles thehigh common mode voltage in the generator.

The modular, series connected converter (Fig.1) consists ofN 3-phase, standard VSC-modules, Fig.6(a). Each module is

id,ref

md2+mq

2 m2dq

Iq-control

Id-control

Flux weakening control

+

+

+

-

-

-

-lim

0

Limiter

m2lim

iq,ref

id

iq

Fig. 7. Block scheme of the implemented flux weakening control schemefor each module.

connected to its own, electrically, magnetically and physicallyisolated stator segment of the generator. Further, the DC-busesof the VSC-modules are connected in series to synthesize theoutput voltage so that an HVDC-output is obtained directlyfrom the converter. Hence, the converter chain provides therequired division of the voltage imposed on the stator seg-ments. At the same time, it also provides a high voltage onthe grid side.

Given 9 modules, the resulting voltage level imposed on eachsegment from a converter module is medium voltage (normallyin the range of 3.3-6.9 kVll,rms), and is therefore within thereach of standard machine insulation.

By incorporating the transformer functionality (voltage step-up) in the generator/converter, the operational benefit of themodular structure of both generator and converter chain canbe exploited fully. Since there are no large components,maintenance and component replacement is largely simplified.

The converter module insulation system will have to be builtup in the same way as the generator segment insulation: Theinternal insulation should be for medium voltage, while theentire VSC-module is insulated from ground.

B. Control system structure

The generator/converter control system is modular, and hasbeen adressed in [11], [7]. A main controller takes care of theturbine operation. In this control unit, set-points for the vectorcurrent control and DC-bus voltage control in the modulesare generated. Each module control (Fig.6(b)) consists of astandard, vector control, position-sensorless control systemwith DC-voltage control for balancing the module bus voltages.The converter control is fully modular, and can functionwith asynchronous communication between main control andmodule. No communication between the modules is necessary.This makes a simple and robust system.

C. Flux weakening control and the series connection

To facilitate overspeed operation in motor drives, flux weak-ening control is normally introduced [22], [23]. Operation atconstant overspeed is not common for wind turbines, sincethis is imposes significant mechanical stress on the turbinestructure. However, transients such as wind gusts or suddenloss of load may result in uncontrolled shaft speed spin up. Toensure full controllability of a PMSG under these transients,

a flux weakening control is introduced to limit the statoroutput voltage within the controller limits. Additioanlly, fluxweakening can also be utilized for loss minimization strategy[24], [25].

Finally, with the possibly different excitations in the large-diameter generator in this work, flux weakening will contributeto the controller saturation occurring at the same speed for allmodules, hence improving the dynamic performance of thesystem.

The flux weakening is realized through injection of a negatived-axis current. The last term of uqs,i in Eq.1 will be reducedby injecting the negative d-axis current, and hence the voltageon the terminals.

uds,i = rsid,i + ωelsiq,iuqs,i = rsiq,i − ωe(lsid,i + ψm,i)

(1)

udq,s,i are the stator voltages in one module, rs stator windingresistance, idq,i stator segment current, ωe electrical, radial fre-quency, ls stator per-unit inductance and ψm,i stator segmentflux.

At the same time, the q-axis current should be limited accord-ing to Eq.2.

iq,i,max =√is,max − id,i (2)

During short transients, is,max will be limited to 20-30 %above nominal current. For steady state, is,max = in

The flux weakening scheme proposed by [23] is implementedin this work (Fig.7). It is based on feedback of the modulationindex, and is entering flux weakening when the modulationindex exceeds the linear region.

IV. SYSTEM SIMULATIONS

A. Simulation model description

Generator model: The stator segments of the AF-IL-PMSGwere modelled using one standard 3-phase machine modelfor each. (The validity of this approach is discussed in [7]).The model utilized was a synchronous machine, with fixedexcitation [26]. In the present design, there are no damperwindings in the generator, and the transient- and subtransientterms were eliminated from the machine model. The machinemodels were interconnected via a first-order model of the shaft,according to Eq.3. In the simulations presented here, threemodules were used. This was chosen because the laboratoryprototype used for experimental validation consisted of threemodules [27].

τmechdnmech

dt= mmech − Σn

i=1mem,i (3)

In Eq.3, mmech is the per unit output torque from the windturbine blade model, and mem,i is the electro-magnetic torque(in pu related to total turbine torque) of the i-th stator segment.τmech is the mechanical time constant, relating the inertia andthe rated power of the turbine. As output, Eq.3 provides the

0

ACDC

ACDC

ACDC

Module 1Udc1

+33.3 kV

-

Module 3

Module 2

///

///

///

SG 3

SG 2

SG1

Ef1

nmech mem1

Ef2

Ef3

nmech

mem2

mem3

nmech

Single mass drive train dynamic equation

Model of 10 MW wind turbine aerodynamics mmech

mem1 mem2 mem3

nmech Udc2

Udc3

Vwind

Fig. 8. Sketch indicating the main electrical and mechanical components andconnections in the simulation model of the modular, series connected convertersystem.

shaft speed, n. The shaft speed is provided as input to thestator segment models.

Converter modules:The VSC-modules were implemented us-ing ideal switches to model the semiconductors. The highvoltage DC-side, and the low voltage AC-side were splitto overcome numerical problems experiences with floatingmodules in PSCAD.

Grid: The grid was modelled as a stiff DC-voltage behind asimple L/R-approximation of the DC-cables. This makes themodel unsuitable for investigations of rapid electromagneticphenomena such as short circuits and high frequency voltages.For the simulations presented here, it was found sufficientlyaccurate.

Turbine model: The wind turbine model in PSCAD wasused to emulate the blade characteristics. It is based on anold turbine model [28], [29]. The parameters of the Cp-curveare therefore not intended for simulation of today’s 10 MWturbines. The response to wind speed and turbine speed werehowever deemed to be sufficiently realistic for this work.

Control implementation: The control system was imple-mented using continuous control blocks, and the measurementswere considered to be ideal.

Parameter deviation: The system was implemented withparameter variations between the stator segments/convertermodules according to Tab.II to emulate a non-ideal systemconstruction.

B. Simulations of flux weakening control in an imperfectsystem

The simulation results for the impact of the flux weakeningcontrol and parameter deviation under DC-bus voltage balancecontrol are presented in Fig.9. The system was first broughtto steady state at nominal wind conditions and turbine speed.At t=65 s, a sudden increase in wind was applied, from 13m/s to 15 m/s. Since the q-axis current was at its maximumlimit, there was no possibility to apply a higher electromag-netic braking force. Consequently, the turbine speed increased

TABLE II. PARAMETER DEVIATIONS FOR SIMULATION. THENUMBERING OF MODULES IS IN ACCORDANCE WITH FIG.8

Module Efficiency [pu] rs ψm

module1 0.98 +25 % 0.95module2 1.0 -25 % 1.05module3 1.0 -25 % 1.05

60 65 70 75 80 85 90

0.8

1

Pow

er/s

peed

[pu]

Time [s]

pturbinenmech

60 70 75 80 85 90

−0.4

−0.2

0

D−a

xis

curr

ent[

pu]

Time [s]

id1id2id3

60 70 75 80 85 900

0.5

1

Q−a

xis

curr

ent[

pu]

Time [s]

iq1iq2iq3

60 70 75 80 85 9010

10.5

11

11.5

12

Time [s]

DC

−vol

tage

[kV

]

Udc1Udc2Udc3

Fig. 9. Simulation results for the turbine, with balanced DC-bus voltagesand a wind ramp from 13 to 15 m/s. From top to bottom: Power/speed of theturbine. D-axis currents. Q-axis currents. DC-bus module voltages.

(before the pitch control was able to limit the power capture).The result of this was increasing induced voltages in thestator segments, and higher flux weakening current demand.This increase in id,ref -amplitude was most prominent in thesegments with the higher stator flux. These have additionalcurrent capacity available, due to the DC-voltage balancecontrol. The q-axis current was therefore not constrained bythe total current limit:

is,lim ≥√i2d,i + i2q,i (4)

Since the current limit was not reached for all modules, thecombination of DC-voltage balance control and flux weakeningproved to be beneficial for the dynamic response of the system.

C. Black start: requirements and feasibility study throughsimulations

Turbine black start capability is necessary seen from a systemoperation point of view. The main benefits of black start are:

• I: Possibility to use the turbines to charge the cablesof the grid in the rare case of a completely black grid.

• II: Make a smooth connection of the turbine to the gridfeasible without application of breakers with large,pre-inset resistors for damping of inrush currents.

Case II will be demonstrated in this section. Case I willnormally be handled by the onshore converter. Eventually,it can be performed by the turbines, using almost the sametechnique as in II.

The smooth connection without pre-insertion of resistors re-quires knowledge of the DC-bus voltage on the grid side ofthe circuit breaker to function properly.

The sequence of a black start for the series connected converteris:

1) Close AC-breakers between stator segments and con-verter modules and initiate the current control withiq,ref,i = id,ref,i = 0.

2) Activate the DC-bus control, with udc,ref = udc,avg.The DC-voltage droop should be inactive at thispoint.

3) Pitch the blades slowly into the wind, allowing theturbine to produce a small torque which acceleratesthe turbine up to nstart

4) Ramp up the DC-voltage to the nominal value for allmodules.

5) When udc,avg = udc,grid close the main turbineconnector

6) Activate the droop control to eliminate steady statecontribution from the DC-voltage controls.

7) Switch to normal operation of pitch control andturbine power control.

The black start requires energy stored in forms of batteries orequivalent for operation of blade pitch and control circuits.

The simulation sequence is shown Fig.10. The process wasinitiated at t=5 seconds when the current control is activated,with zero as reference. The turbine was pitched in to the windone second later, and the rotor slowly started to accelerate.This resulted in an induced stator voltage, and hence a currentcharging the bus capacitors. At t=9 s, the DC-voltage balancecontrol was activated, and a ramp reference with nominal gridvoltage as final operation point was applied. This resultedin a positive balance current contribution. When the nominalvoltage was achieved, the grid connector was closed (t=14.97),and the bus voltage control was switched to normal mode,where the droop control eliminates the net contribution fromthe DC-voltage controller outputs. Eventually, the turbine con-trol shifted from start-up mode to normal mode. This occurredaround t=25 s. 1

D. Open cable faults

In a situation where the grid connection is lost, the excessenergy captured by the turbine will have to be absorbed locally.

1The turbine model in PSCAD is not suitable for black start simulations,since it is only implementing an expression for the power which is, amongstothers, based on the speed as input. Its output at zero speed is strictly zero.A speed ramp was therefore utilized for initial phase, and when all transientswere eliminated, the model was switched to turbine model.

10 20 30 40 50 600

0.5

1

Pow

er/s

peed

[pu]

Time [s]

pturbinenmech

10 20 30 40 50 600

5

10

Time [s]

DC

−vol

tage

[kV

]

Udc,1Udc,2Udc,3

10 20 30 40 50 60−0.5

0

0.5

Bal

ance

curr

ent[

pu]

Time [s]

iq,bal,1iq,bal,2iq,bal,3

10 20 30 40 50 600

0.5

1

Q−a

xis

curr

ent[

pu]

Time [s]

iq,1iq,2iq,3

Fig. 10. Simulation results for the black start procedure for the modular, seriesconnected VSC tied to a HVDC-grid. From top to bottom: Turbine speed andpower. DC-bus voltage. DC-bus voltage control current output. Simulated iq,ifor the stator segments.

There are two sources which can absorb the energy in thepresent system: 1) the rotational mass of the turbine. 2) thecapacitive energy storages of the converter modules. Neitherof these can accept large energy quantities: By protecting thebus capacitors from overvoltages, the turbine will spin up.This will result in high mechanical stresses on the structure.Limiting the turbine speed will lead to overvoltages on the buscapacitors, which may lead to destructive failures. The powercapture of the turbine will decrease if the speed is allowedto increase, bringing the turbine out of the optimal powerpoint. However, the inertia is too high for this to aid whendealing with the short time constants of the power electronicconverters. Consequently, additional protection measures arenecessary.

The most common way of protecting a power electronicconverter is a chopper circuit on the DC-side of the converter.In the investigated converter topology, there are two possibleconfigurations: either one large chopper circuit (Equivalentto the HVDC-converter choppers), or one per module. Theadvantage of the first is the minimization of component count,while the latter respects the modular approach, and allowsfor lower voltage ratings. An additional benefit of the modulechopper is that it can protect each single module. A loss of loadwas simulated for different configurations and voltage levels to

49 9 49 95 50 50 05 50.1 50.15 50.210

11

12

13

14

15

Time [s]

DC

−vol

tage

[kV

]

Case1Case2Case3Case4

49 9 49 95 50 50 05 50.1 50.15 50.2

−1

−0.8

−0.6

−0.4

−0.2

0

0.2

I q,ba

l[pu]

Time [s]

Case1Case2Case3Case4

49 9 49 95 50 50 05 50.1 50.15 50.2

−0.5

0

0.5

1

mem

[pu]

Time [s]

Case1Case2Case3Case4

Fig. 11. Simulated system response to a sudden loss of grid at t=50 s forthe different protection strategies. From top to bottom: DC-bus voltages inone module, output from DC-bus voltage controllers, Electromagnetic torque.(Cases according to above defined list)

assess the energy dissipation requirements for each solution.

• Case 1: Control actions to limit the bus voltage [30].DC-voltage limit: 1.1 pu.

• Case 2: Central chopper. DC-voltage limit: 1.1 pu

• Case 3: Chopper in each module. DC-voltage limit:1.1 pu

• Case 4: Chopper in each module. DC-voltage limit:1.2 pu

In addition to the module control, a fail-safe emergency pitchwas applied. This ensures reduction of energy capture fromthe turbine, and eventually brings the rotor to standstill. Theemergency pitch is limited to 6 per second due to mechanicalconstraints imposed by the blades. Normally, there wouldalso be included some form of mechanical brake. This is notincluded in this study.

The requirements for the different chopper configurationsare given in Tab.III, where Rchopper is calculated based onnominal power dissipated at Udc,max.

Case 1 is used as reference. The torque reference is multipliedby a gain inversely proportional to the DC-bus voltage. At

TABLE III. RELATION BETWEEN DC-BUS ENERGY, CHOPPER ANDTIME CONSTANTS

Udc,max [pu] Rchopper

1.1 (central) 1210 Ω1.1 (module) 136 Ω1.2 (module) 160 Ω

49 50 51 52 53 54 550.2

0.4

0.6

0.8

1

1.2

1.4

Turb

ine

spee

d[p

u]

Time [s]

Case1Case2Case3Case4

Fig. 12. Turbine speed response to the loss of grid; compared for the differentprotection strategies.

nominal voltage, the gain is 1, while at the voltage limit, thegain is 0.

The simulation results are presented in Fig.11, and key numer-ical values in Tab.IV.

From the results, it can be concluded that the reference casedo not limit the system in a satisfactory manner, as indicatedin the beginning of this section. The turbine speed is increasedup to 20 % above the rated before it is limited (Fig.12, whilethe peak DC-voltage is 15 %. This increases the demands onthe system level. The three cases with chopper circuit includedare all capable of limiting the DC-voltage according to theirdefined limits. Additionally, the turbine speed is limited to lessthan 10 % overspeed. Hence, the chopper circuit, independentof location in the converter, will contribute to improved faultconditions for the system.

The energy dissipation difference is significant when compar-ing the two cases with 10 % overvoltage limit. The case withmodular chopper is demanding 13.9 % more energy dissipa-tion. The reason is that the DC-bus voltage is maintained atthe same level, while the module bus voltages are allowed tovary slightly with central chopper. It is therefore of interest toinvestigate the dissipated energy for each module. Tab.V liststhe dissipated energy by module. A major difference betweenthe modules is observed. Hence, the total chopper design in themodular case (1.1 pu voltage limit) should be 7.32 MJ whenconsidering that the system imperfectness is not possible toaccount for in the design phase. This corresponds to an over-rating of 37 % compared to a central chopper circuit.

TABLE IV. DISSIPATED ENERGY IN THE DIFFERENT CASES

System config. Energy dissipated Max overspeed Max voltage

Case 1 - 1.20 12.8 kVCase 2 5.35 MJ 1.07 12.36 kVCase 3 6.09 MJ 1.06 12.23 kVCase 4 3.91 MJ 1.09 13.33 kV

TABLE V. DISSIPATED ENERGY IN BUS CHOPERS

System config. Module 1 Module 2 Module 3

Case 3 2.44 MJ 1.82 MJ 1.82 MJCase 4 1.62 MJ 1.14 MJ 1.14 MJ

V. IMPACT OF HIGH TURBINE VOLTAGES ON THE TOTALSYSTEM DESIGN

The system voltage level (110 kVdc) which is used for theinvestigations is chosen based on the potential of the genera-tor/converter technology. It is not to be understood as a readilyoptimized solution. 100 kV may prove too high for distributiongrids, and too low for long distance transmissions.

An additional issue which needs commenting is the volt-age/power rating of each converter module in the modularseries connected converter. 11 kV DC-link is in the upper rangeof medium voltage drives segment. This is normally reservedfor higher rated power than 1 MW. To really benefit from massproduction, more converter modules with lower voltage levelscould provide a better solution.

In general, the voltage levels considered here requires highvoltage components for relatively low power ratings. Such acombination is rare, and therefore might prove expensive dueto the lack of interest for components in this segment.

An additional concern is the protection of the system againstgrid faults in a 100 kVdc-grid. Although there exists proposedsolutions for HVDC-breakers, no installations in real gridshave been demonstrated yet. Additionally, their cost and com-plexity might make these uneconomical for a small unit as a10 MW.

The optimization of voltage levels is not considered further.This is an economical as much as technical optimization, andlays outside the scope of this work.

VI. CONCLUSION

The analysis of a modular series connected voltage sourceconverter has been presented in this paper. The emphasis hasbeen on transient performance outside the normal, nominaloperation. First, an overview of existing converter concepts fortransformerless systems has been presented. Then, the investi-gated system is presented, and a flux weakening scheme wasintroduced to the modular converter control system. The com-bination of this flux weakening control and system parameterdeviation was investigated through simulations. An improvedsystem dynamic performance was found due to combinationof the the balance control system and flux weakening.

Further, the feasibility of black start of the turbine withoutadditional hardware was demonstrated. Finally, the loss of load(open-cable fault) was investigated. A major impact of thelocation of protection device (chopper circuit) on the energydissipation requirements was found. One central chopper islargely beneficial in terms of energy and component count.However, the voltage levels will be easier to handle if achopper is included in each VSC-module.

In the end, a brief discussion on system power- and voltagelevels is included.

ACKNOWLEDGMENT

The authors would like to thank SmartMotor AS for their con-tribution to the project. The work is conducted as a part of theNorwegian Research Centre for Offshore Wind Technology,NOWITECH.

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