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AC/DC isolated boost converter for direct connection of wind

generators to HVDC cable

Bruno Miguel Dias Raposo,

Instituto Superior Técnico – UTL,

Lisboa, Portugal

bruno.raposo@ist.utl.pt

Abstract – Wind generation seems to be one of the most

relevant sources of renewable energy, providing an

alternative to conventional energy sources. However, most of

the places onshore that assure an economic viability has been

taken or are restricted by environmental organizations.

Furthermore, for offshore wind farms there are no obstacles

to slow down the wind speeds, making the sea a suitable

place to install wind turbines. However, one of the big

challenges can be the long distance to the onshore power

substation, in order to take advantage of the strong wind

speeds far offshore. Therefore, an alternative to the AC

power transmitting technique is needed, because when the

cables transmit large amounts of AC energy they generate

significant amounts of reactive power which have to be

compensated with expensive equipment (STATCOMs or

capacitor banks for example). Dielectric losses or the need to

use several cables in parallel are also constraining factors.

The DC technology does not have these problems, becoming

an option to be taken in account when we want to link

offshore wind parks to the onshore grid. The work proposes

a power conversion to interface the offshore wind park

turbine to a DC cable using an innovative solution that

associates an isolated Boost Converter with a high frequency

tri-phase transformer (2 kHz), followed by a series

association of diode rectifiers. This association will be

studied, analyzed and reviewed in terms of electrical

viability for this application.

Key-words: Offshore wind parks, HVDC, Active-Clamp Full-

Bridge Boost Converter, PMSG, wind turbine clusters.

I. INTRODUCTION

Our social model is firmly dependent of the electricity

production which has raised several issues over time. The

emissions of CO2 to the atmosphere by burning fossil

fuels or the continued use of this raw material to

exhaustion are just some of them. In this context

renewable energies can matter. Wind energy has been

gaining ground in the electrical power generation. In

nominal operation, it is possible to produce large amounts

energy. On the other hand the high volatility and

irregularity of wind, makes wind an almost non

predictable energy source with low duty factor. So it is

always necessary to build thermal power plants to work as

a "backup". However, the winds are more stable and

predictable offshore, therefore offshore wind turbines can

be more successful. There are some challenges like

extreme environment, corrosion of the materials due to the

salinity, large distance to shore, among others. However

there are advantages as well. Europe alone has a large

wind potential to be explored, reduced visual pollution

and existent infrastructures to be used from the oil

platforms out of service [1][2]. In Europe, the United

Kingdom was the one that installed more offshore wind

turbine in 2011, followed by Denmark and Netherlands

[3]. Portugal installed a 2 MW wind turbine for testing,

the Wind Float project. Normally, these wind turbines

have a 50/60Hz transformer. However, in this article it

will be used a high frequency transformer (2 kHz) in order

to be smaller/lighter than a transformer of the same

power. This is ideal for applications that require

transformers inside the turbine itself. The system must

increase (using some steps) the voltage supplied by the

generator to transmission levels in order to minimize

losses in the HVDC cable. Generating high voltages

through the use of power converters has its drawbacks.

The semiconductors used will have to bear this same

voltage. The association in series of semiconductors is an

option, however risky because it may cause the failure of

switches due to poor synchronization or other issues.

Some solutions on the market circumvent these problems,

but require a substation at sea due to its size and weight.

What is proposed is a wind park arrangement and power

converters able to convert the 690 V ac from the generator

to a DC voltage (50 kV), without using series connection

of controlled power semiconductors.

Figure 1 - System in study

II. TRANSMITION SYSTEMS FOR OFFSHORE

WIND PARKS

A. Solutions Available

The High Voltage Alternating Current (HVAC) is the

most common way to transmit energy and the most used

around the globe. For this application, this technology is

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limited by the distance of the park to shore.

Figure 2 – Electrical layout – HVAC [4]

The High Voltage Direct Current – Line Commutated

Converter (HVDC-LCC) is well known and is used to

transmit energy in DC. However, it is more expensive and

it is only economic viable for large amounts of energy and

long distances.

Figure 3 - Electrical layout - HVDC-LCC [4]

The High Voltage Direct Current – Voltage Source

Converter (HVDC-VSC) is the most recent of these

technologies, introduced to the market by ABB and

Siemens. It uses IGBTs and enables the control of active

and reactive power produced/consumed by the

installation.

Figure 4 - Electrical layout - HVDC-VSC

B. Proposed solution

The proposed structure is based on the association of

turbines in series or in parallel, forming clusters [5]. If we

choose to form clusters of turbines in parallel we must

have a transformer to raise the voltage levels of the

transmission or even to raise the voltage within the park.

The low voltage (high current) in the park and the

platform required by the fact of having a transformer will

increase the cost of the installation. The advantages are

that the transmission losses are further reduced since we

have high voltage, we have no reactive power and it is a

mature technique used over the years. An alternative to

the internal network of the park in AC and so the

transformer, consists in having the converters raising the

voltage to transmission levels. This assumption may not

be easy or even achievable due to limitations of the

semiconductors.

Figure 5 - Parallel clusters of turbines with AC internal network

and DC transmission [5]

If we choose to put turbines in series [5][6], we can

achieve higher voltage levels without an external

transformer. The losses are substantially reduced in the

park because the voltage is higher which result in a better

efficiency and less material needed. However losses in

converters remain significant, but this depends in the type

of converter used.

Figure 6 - Series clusters of turbines with DC internal network

and transmission

In this paper, for a typical wind farm of 100 turbines with

2MW each, it was chosen to place four close turbines in

series and it was assumed that each have in their terminals

50 kV DC, being 200 kV the transmission voltage. No

guarantee that they are optimal voltage levels, but these

values are proper to show the concept of system

operation.

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Figure 7 - Proposed structure for the wind park

III. POWER CONVERTERS

A. Tri-phase association of DC-DC boost converters,

isolated with high frequency tri-phase transformer and

half-bridge rectifiers

This work studies a combination of three-phase DC-DC

boost converters with clamp circuits isolated by a high

frequency (2 kHz) three-phase transformer. In each block

the commands for the semiconductor are 120º advanced to

each other in order to obtain a balanced three-phase

system. The complete system is shown in Figure 9.

Connected to the transformer there are three half-bridge

rectifiers connected in series to obtain the 50kV DC

voltage. With this topology is intended to prevent the

placement of power semiconductor devices in series [7].

The input DC voltage is obtained by a three-phase

rectifier switching forced placed after the generator.

Figure 8 - Input voltage building

Figure 9 - Tri-phase association of DC-DC boost converters,

isolated with high frequency tri-phase transformer and double

half-bridge rectifiers

B. Active-Clamp Full-bridge boost converter

This Active-Clamp Full-bridge boost converter must raise

the AC voltage (690V) from the generator into DC

voltage (50kV) step-by-step. The rectified voltage (1.2

kV) of the permanent magnet synchronous generator

(PMSG) is obtained so the maximum possible power is

extracted using a unity power factor rectifier, described

later. Each block of Active-Clamp Full-bridge boost

converter, shown in Figure 10, is constituted by an

inductance L for an intermediate storage of energy which

then is delivered to the transmission cable. Below is a full

bridge to do the inversion from DC to AC.

Figure 10 - Active-Clamp Full-bridge boost converter,

transformer and double half-bridge rectifier

The semiconductors have the following behavior:

(1)

As the transformer is not ideal, the stray fluxes are

represented by a coil (primary), as shown in Figure

10, which cannot be neglected, since when all

semiconductors in the bridge are conducting, and

will be in series. Since the current will not be the same,

high voltages will be generated. For the circuit to operate

without these issues, we need a capacitor that will load

until the current in is equal to the one in . It

discharges when the voltage at its terminals is higher than

the primary of the transformer. However, in the situation

when all semiconductors are conducting, the capacitor

would be short-circuited. So it is necessary to place a

diode before Cc and a controlled semiconductor Sc

allowing the discharge of the capacitor. It will be ON

when S1, S2, S3 and S4 are not connected simultaneously.

As this is a boost converter, the equations in [8] are valid

to size the components. To control the current in the

inductance L, the control scheme in the Figure 11 is

presented.

Figure 11 - L current linear control

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Table 1 - Parameters

Taking into account the losses in the system, we will have

an efficiency of 96%, which is acceptable. To command

the semiconductors we used the Pulse-width modulation

(PWM) technique because the times of conduction must

be equal in all the semiconductors so we don’t have any

DC component that can saturate the transformer.

Figure 12 - Current wave forms - boost converter

Figure 13 - Voltage wave forms - boost converter

As we can see in the Figure 13, the voltage is boosted

step-by-step along the components of the boost converter. The Figure 14 shows the controlled DC voltages at

several levels of the system. Compared with a standard

rectifier with diodes, this converter has several DC levels

of voltage, allowing a phased voltage boost.

Figure 14 - Controlled DC voltages of the boost converter

C. Three-phase rectifier switching forced

This converter will operate as an elevator three-phase

rectifier, in order to transform AC magnitudes generated

by the PMSG (690 V compounds) to DC magnitudes of

higher value (1200 V).

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Figure 15 - Magnitudes generated by the PMSG rectification

The command chosen was the Nonlinear Vector Control

of Three-Phase Currents [8] which is based on comparing

currents in αβ coordinates with the reference currents and

using the sum of two hysteretic comparators, considering

the component as suggested in [9]. As we can see

in the Figure 16, the single phase voltage and the current

are in phase opposition, which means that the generator is

delivering energy to the system.

Figure 16 - Power, single-phase voltage and current – PMSG

Figure 17 - Output currents from the PMSG

To control the voltage 1,2 kV, the control scheme in the

Figure 18 is proposed.

Figure 18 - Input voltage 1,2 kV linear control

IV. WIND TURBINE CONTROL

A. Gear-Box

The link between the turbine and PMSG was made using

a gearbox, modeled by a gain. On one side we have about

20 rpm and the gearbox will have to increase this speed to

approximately 1500 rpm, the speed that most generators

require to produce electricity. The parameters used are

from [10].

(2)

( ) (3)

(4)

B. The Generator

The generator chosen for the wind turbines was the

Permanent Magnet Synchronous Generator (PMSG). This

choice is based on their viability and low maintenance.

The machine dynamics can be represented by a bi-phase

system, represented by two axes - the d axis aligned with

the rotor position and the q axis that lies ahead 90º from

the d axis [9].

Figure 19 – abc and dq coordinates in an electrical machine [9]

( )

( )

(5)

[ ( ) ] (6)

(7)

Table 2 - PMSG specifications

C. Wind Modulation

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The kinetic energy available in the wind, combined with a

mass of air m that moves at constant and uniform speed u,

in a wind turbine is given by [11]:

(8)

Considering , the power available in

the wind is:

(9)

The Bezt limit, considering β the pitch angle, is given

by:

(10)

(

)

(11)

(12)

(13)

(14)

Based on [12], we can draw the turbine characteristics for

different wind speeds and a null pitch angle.

Figure 20 - β =0 turbine characteristics for different wind speeds

For the speed control of the wind turbine, the optimal

generator speed that grants the maximum power extracted

is:

(15)

The optimal value can be followed by the system using

the control scheme presented in the Figure 21.

Figure 21 - PMSG speed control

For the torque control of the wind turbine, the optimal

generator torque that grants the maximum power extracted

is:

(16)

The optimal value can be followed by the system using

the control scheme presented in the Figure 22.

Figure 22 - PMSG torque control

V. WIND PARK DC VOLTAGE CONTROL

A. HVDC Cable

The type of cable chosen for the connection between the

wind farm and the shore was the XLPE (Cross-linked

polyethylene). This is the type of cable used in most of the

applications at sea, having high elasticity, resistance to

abrasive elements and with extreme viability. In order to

calculate the losses in the HVDC cable, we must choose

the most appropriate for the situation. So it is necessary to

estimate the current that pass through it in nominal

conditions.

(17)

Consulting [13], we concluded that a 1400 mm2 cable is

suitable for this case because it has a rated current of

1600A, giving the system a good safety margin. Their

resistance per km in length is then:

(18)

Also from [13] we know that the cable have an

inductance . The distance that

the wind farm would be from the coast with this system

must be such that the HVDC had more advantages than

the HVAC. According to [14], when comparing HVDC

technology with the HVAC, the "break-even-point" will

be at around 100 km. From this distance it will be more

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cost effective to install an HVDC system than a HVAC

one. This point varies from system to system because it

depends on the potency of the park, power electronics

used, types of cables, hand labor, required infrastructures,

among others.

Figure 23 - break-even-point [14]

(19)

The losses in the cable can be estimated by:

(20)

B. Cable voltage control 200kV

The architecture and inverter operation on land that will

connect the cable to the HVDC grid are not in the goals of

this article [15]. However, controlling the DC voltage for

200kV must be done, since this is essential to the wind

farm. To simulate the control of this magnitude we

decided to consider the control scheme in the Figure 24.

The wind farm generates a current which is divided

between a condenser and the inverter connected to the

grid.

Figure 24 - 200 kV voltage linear control

Figure 25 - Transmission voltage in HVDC cable

VI. RESULTS

The results are now presented.

Figure 26 - Wind speed profile [19]

Figure 27 - Reference and electromagnetic torque

Figure 28 - Reference and generator speed

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Figure 29 - Power comparison between torque and speed control

VII. CONCLUSIONS

This paper investigated the possibility of using a three-

phase boost converter isolated with a high frequency

transformer and half-bridge double rectifiers associated in

series to transform 690VAC from the PMSG to the output

50kV DC of a wind turbine with a single converter and no

power semiconductors in series. The most important

advantage of this system is that no substation at sea is

required, reducing dramatically the installation cost.

Thanks to its electrical architecture, we can increase the

voltage to transmission levels, with equipment that fits

perfectly within the framework of the turbines. The

simulation results match the theoretical ones,

demonstrating that the system is properly designed and

with optimal conditions. Accounting system losses,

approximately 86.7 kW, we conclude that the efficiency is

about 96%, which is acceptable given the amount of

power electronics involved. Losses are well distributed

among the various elements of the system. The

simulations show a stable DC voltage in both the internal

network of the park as in the transmission cable, with no

significant ripples. The system responds very well to both

torque and speed controls, being able to follow the

optimal references, depending on the wind speed

occurring in a given instant. We also conclude that the

speed control takes over 7% more power at the startup

than the torque control. Within thirty seconds of

simulation the turbine can generate about 8.1 kWh of

energy.

VIII. REFERENCES

[1] Silva, Filipe, “Offshore wind Parks Electrical

Connection”, IST, Julho de 2008.

[2] Ackermann, Thomas, “Wind Power in Power

Systems”, Royal Institute of Technology, Sweden, 2005.

[3] J. Wilkes, J. Moccia, A. Arapogianni, M. Dragan, N.

Plytas, A. Genachte, J. Guillet, P. Wilczek, “The

European offshore wind industry key 2011 trends and

statistics”, The European Wind Energy Association

(EWEA), January 2012.

[4] Ulsund, Ragnar, “Offshore Power Transmission –

Submarine high voltage transmission alternatives”,

NTNU, 2009.

[5] A. Garcés, M. Molinas, “Coordinated control of

series-connected offshore wind park based on matrix

converters”, Wind Energy, 2011.

[6] Anish Prasai, Jung-Sik Yim, Deepak Divan, Ashish

Bendre, Seung-Ki Sul, Frank Kreikebaum, “A New

Architecture for Offshore Wind Farms”, Georgia Tech,

Atlanta, Seoul National University Korea, DRS

Technologies.

[7] Martins, H., “Interactive anti-islanding inverter”, IST,

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[8] Alves da Silva, J. F., “Sistemas de Energia em

Telecomunicações: Textos de Apoio”, DEEC, Instituto

Superior Técnico.

[9] Domingos Marques, G., “Controlo de Motores

Elétricos”, Fevereiro 2007.

[10] Ming Yin, Gengyin Li, Ming Zhou, Chengyong

Zhao, “Modeling of the Wind Turbine with a Permanent

Magnet Synchronous Generator for Integration”, IEEE,

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[11] Gameiro de Castro, R. M., “Uma Introdução às

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hídrica”, Lisboa: IST Press, 1ª Edição.

[12] Catalogo V80-2MW VESTAS.

[13] XLPE Submarine Cable Systems – User’s Guide,

Rev 5, ABB

[14] Bresesti, Paola, “HVDC Connection of Offshore

Wind Farms to the Transmission System”, IEEE, Março

2007.

[15] Nunes, H., “Nova topologia de conversor multinível

para parques eólicos marinhos”, IST, Setembro de 2010.