Multi-megawatt wind turbine converter configurations suitable for off-shore applications,

combining 3-L NPC PEBBs

Javier Chivite-Zabalza* Igor Larrazabal *

Ignacio Zubimendi * * Ingeteam Power Technology S.A. - Technology

Parque Tecnológico, 110, 48170, Zamudio, Bizkaia, Spain

Sergio Aurtenetxea * Mikel Zabaleta **

** Ingeteam Power Technology S.A. - Energy

Avda. Ciudad de la Innovación, 13, 31621 Sarriguren (Navarra) - ESPAÑA

Abstract— This paper presents two power converter configurations, suitable for an off-shore wind turbine in the range of 6 MW. These, which may either drive a Permanent Magnet Generator (PMG) or an Electrically Excited Synchronous Generator (EESG), are built combining a number of water-cooled, 3.1 kV, 3-Level Neutral Point Clamped (3L-NPC) Power Electronic Building Blocks (PEBB) that are based on High Voltage IGBTs (HV-IGBT). The first configuration employs two electrically isolated conversion lines, achieved by having two independent stator windings, where the converter outputs are connected in parallel at the grid side by means of two sets of line inductors. The second configuration employs a series connection, achieved by having open-end terminal connections, both at the machine and grid-side transformer. The paper begins by describing the PEBB modules, followed by the detailed description of the proposed configurations. Subsequently, a detailed comparison considering the main features for an off-shore application is presented.

Keywords— Power conversion line, Power Electronic Building Block - PEBB, Off-shore wind turbine configuration, Parallel power converters, Series power converters.

I. INTRODUCTION The off-shore wind installed power is increasing

dramatically during last years [1],[2]. This is due to factors such as the higher and steadier wind speed than that on shore, the lack of wind resources in densely populated areas and the reduced environmental impact such as audible noise and visual impact [3]. An example of these developments is the many wind-farms being planned in the UK, Germany and France in the next five years [1],[2].

Examples of current commercial off-shore wind turbines include the AREVA WIND M5000, a 5 MW, Permanent Magnet Generator (PMG), using Medium Voltage (MV) 3.1 kV Full Converter (FC) technology; the REPOWER 5M, a 5 MW Doubly Fed Induction Generator (DFIG) using Low Voltage (LV) 690 V converter technology; the SIEMENS SWT-3.6 MW turbine, based on a FC asynchronous Induction Generator (IG) using a 690 V LV converter; the ALSTOM Haliade 150, a Direct Drive PMG 6 MW generator operating at

900 V, and the VESTAS V90 3M and V112 3 M, which uses a 3 MW PMG, FC generator. Further developments are currently underway from these manufacturers, aiming to increase the output power in the range of 6 MW to 10 MW [4]-[8]

In order to adapt to the different application requirements, there is the need to develop and combine standard Power Electronic Building Blocks (PEBBs), well established in applications such as medium voltage inverter motor drives and used in the wind, marine, steel and grid industries, in an intelligent and cost effective way. Some of the main features that these PEBBs are required to fulfil include power density, efficiency, modularity, scalability, standardization, supply chain optimization-versatility, maintainability, and overall development time [9].

In this paper, two advanced wind turbine converter configurations are proposed, based in a newly developed family of 3.1 kV, water-cooled, 3-Level Neutral Point Clamped (3L-NPC) inverter PEBBs, that use the latest 4.5 kV High Voltage IGBT technology [10]-[12]. They target a 6 MW Permanent Magnet Generator (PMG) or Electrically Excited Synchronous Generator (EESG). These are two possible configurations out of the many more that serve the purpose of illustrating the high flexibility offered by this new modular converter family. The first configuration is achieved by combining two Power Conversion Lines (PCLs), electrically isolated by means of separate dc-buses and two sets of independent stator machine windings. This configuration has excellent redundancy features and a good power quality on the grid side. The second configuration is achieved by combining two PCLs in series, by means of having open-end terminal connections at the grid transformer and machine side. This configuration doubles the output voltage at both the machine and grid side, with the subsequent overall efficiency improvement and increases the number of voltage levels in the waveform from three to nine. This is done at the expenses of sacrificing redundancy.

The paper begins by describing the PEBB modules. Then, the advanced converter configurations for wind turbines are explained, and some of the main features such as volume,

2635978-1-4799-0336-8/13/$31.00 ©2013 IEEE

POWER STACK CABINET 1 3L NPC Topology 2 HV-IGBT & Diodes 3 3,3kV/4,16kV 4 Removable PEBB 5 1 Chopper on Top 6 6 Phases (2 Inverters) 7 Non de-ionized water

Fig. 1 Main Power Stack characteristics for the MV100-INGECON WIND converter system

Fig. 2 6 MW, 3,1 kV wind turbine converter using two power conversion lines, connected in parallel at the grid side and having two electrically isolated generator stator windings

weight, power density, scalability, grid code and Low Voltage Ride Through Compliancy and scalability, power quality of the output waveform and filter size, redundancy and efficiency are discussed. Subsequently, a comparison between the two solutions is carried out, mainly focusing on the converter voltage, rated power, power quality of the output waveform and filter size, redundancy and efficiency. This is followed by the main conclusions.

II. POWER ELECTRONIC BUILDING BLOCK –PEBB - A PEBB that is suitable for this type of applications is the

MV100 of Ingeteam and is shown in Fig.1 [10]. The PEBB has been conceived considering different concepts which help with the electrical cubicle design to achieve a reliable, modular, easy to maintain, compact and flexible product. The PEBB Module is formed by a water cooled heat sink where 4 IGBT and 2 Diodes are mounted on the top side. The aluminum water cooled heat sink is provided with quick-release and non-spill connectors.

The power busbar connects all the semiconductors between them and additionally permits to have externals connections of the PEBB:

• Main DC Bus connection (P, 0V and N). • Output AC phase connection. • Current measurement signal. • Fibre Optics signals • Driver Power Supply connections. • Temperature measurement. The PEBB, which could be available with an output voltage

of 3.3kV or 4.16 kV, as soon as the 6 kV HV-IGBT technology becomes available, can be easily plugged in the rear fast connections (DC Busbar and AC connection) to achieve a modular system configuration. For instance, a complete back-to-back inverter system would comprise 6 Phase PEBB and 1 Chopper PEBB, all integrated in a single electrical cubicle.

The position of the different elements have been chosen depending on the cabling interconnection, water cooling circuit access, weight, accessibility and easy maintainability of whole system. Fig. 1 shows main application and mechanical characteristics of the Power Stack.

An interesting feature of the MV100 family, which

facilitates the task of combining several inverter poles in a coordinated manner, is its distributed converter control structure. This is formed by a Central Control Unit (CCU) and by a set of Power Management Modules (PMM) attached to every three-phase inverter. The CCU performs the overall converter control current and voltage loops, whereas the PMMs provide the second level of protection, the modulator, and the basic switching logic for each converter pole. The PMMs communicate with the CCU via a fast fibre optic link, allowing for a flexible and reliable system configuration.

III. ADVANCED CONVERTER CONFIGURATIONS FOR WIND TURBINES BASED ON PEBB

The wind turbine configurations are based on two PCLs. The first one is based on a multi-winding generator which avoids circulating currents among the converters on the machine side. In addition, separate dc-buses are considered, avoiding the circulation of zero sequence current on the grid side [13]. On the other hand, a one-winding generator with open-end terminal connections at the machine side and grid transformer is considered. In a similar way, this configuration avoids circulating currents among the power conversion lines.

Table I shows an example of the small differences from the point of view of generator’s manufacturer for both solutions focused on PMG technology. The main difference between both technologies is the insulation requirements as the voltage levels increases.

TABLE I -6 MW PMG GENERATOR TECHNOLOGIES

V [kV]

N. of windings

Power [MW]

Technology Weight [p.u]

Efficiency [%]

Cost* [p.u]

3.3 2 6 PMG 1 >98.5 1 6.6 1 6 PMG 1.02 >98.5 1.07

*Estimated cost for a 250 rpm generator

A. Parallel connection of two conversion lines As already explained, this configuration, seen in Fig. 2,

combines two power conversion lines, electrically isolated by means of separate dc-buses and two sets of independent stator machine windings. Since the machine stator windings are electrically isolated, the converters on each PCL can operate independently, while equally sharing the power. Moreover, this solution offers good scalability features, achieved by adding more parallel-connected PCLs.

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Fig. 3–PWM wave profiles considering the parallel configuration

Fig. 4 Current sharing on each PCL and the resulting current waveform

Fig. 5 Current harmonic spectrum and grid code limits

Fig. 6 Ingeteam’s power conversion cabinet for 6 MW, 3,1 kV

On the grid side, several harmonic cancellation or doubling techniques can be used to enhance the purity of the output waveform. For instance, if carrier-based triangular sine wave Pulse-Width modulation (PWM) modulation is used, the switching frequency of the output can be doubled if the two level-shifted carrier waveforms used to produce the voltages U1 and U2 are phase-shifted by 180º, with the reference waveform being common to both inverter poles. This is illustrated in the waveforms of Fig. 3. Moreover, similar results can be obtained by using well known Space Vector PWM (SVPWM) modulation techniques [13]-[15].

Since a slight mismatch in the value of the two inductors is likely to be found in practice due to manufacturing tolerance reasons, each grid side inverter will have its own current control to ensure a good current sharing. Although any deviation in the required output waveform may impair slightly the harmonic cancellation, in practice this cancellation has been found to be very good. The harmonic cancellation allows reducing the filter size. However, to support redundant operation, the line filters are designed to meet power quality specifications when only one PCL is in operation. That is, they consider the mitigation of current harmonics around all the switching frequency side-bands.

Fig.4 shows the current sharing between two converters and the resultant line current. The grid code fulfilment is shown on Fig. 5 where IEEE519-92 and BDEW limits are above grid side current harmonic spectrum.

One of the most interesting features for an off-shore application is the redundancy levels that are offered. In the case of failure of a conversion line, the system can continue operating at half the rated output power. This is better quantified in the comparison section.

A picture of a complete converter system based on the parallel configuration is shown in Fig. 6. This product offered by Ingeteam contains all the required elements for a real wind turbine off-shore application, including the water cooling unit, converter control, power stacks, line and machine side filters and switchgear.

B. Series connection of two conversion lines In this second configuration, shown in Fig. 7, the two

conversion lines, which have separate dc-buses to avoid the flow of common-mode currents, are connected in series or in a so-called H-bridge fashion, by supplying a machine and grid-side transformers with an open-end terminal configuration.

In doing so, the output voltage is twice the equivalent line-to-neutral voltage of the converter system in Fig. 2. Moreover, the number of voltage levels increases from three to five, improving the purity of the output voltage and current significantly [15], see Fig. 8. For instance, if carrier-based triangular sine wave Pulse-Width modulation (PWM) modulation is used, the switching frequency of the output can be doubled if the two reference waveforms used to produce the voltages U1 and U2 are phase-shifted by 180º, whilst sharing the same level-shifted carrier waveforms. Since the grid or machine-side output current flows through both inverters alike, only a single current control is required. Moreover, since the

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Fig. 7 –6 MW, 3,1 kV wind turbine converter using two power conversion lines, connected in series by having open-end machine and grid transformer terminals.

Fig. 8– PWM wave profiles considering the series configuration

Fig. 9 Current sharing by both PCLs and the resulting current waveform

Fig. 10 Current harmonic spectrum and grid code limits- Series configuration harmonics are cancelled out at its source, that is, at the output

voltage, this cancellation is independent of any possible mismatch in the values of the line inductors. Besides, due to the series connection, the line inductors are designed for a half of the required output equivalent inductive value with respect to parallel configuration.

An example of this can be seen in the waveforms in Fig. 8 and Fig. 9. The physical appearance of the grid current waveform is very similar to that already seen in Fig. 4. This configuration complies with the same grid codes that are already shown in Section III.A, see Fig. 10.

In spite of the extra degree of freedom that this configuration leads, each power conversion line must work in a coordinated way. In fact, each one supplies the half of the nominal power but whole energy is managed considering a single power conversion system without any redundancy features. In a similar way, scalability issues are not evident under this situation.

The appearance of the whole power converter system is very similar to that shown in Fig. 6 for the parallel configuration.

IV. COMPARISON REVIEW This section aims to compare the main features of the two

already presented parallel and series configurations respectively. The requirements on THD, Low Voltage Ride Through (LVRT), grid code compliance and P-Q operating range are all fulfilled, but are not discussed in further detail for brevity reasons.

A. Converter Voltage

The parallel configuration is characterized by the use of each power conversion line in an independent way in the 3.1kV voltage levels. It offers an excellent performance and a good power quality both the grid and the generator side. However, the series configuration presents some advantages with respect to this configuration. These include a higher voltage, between 1.15 to 2 times higher, in the transformer and machine windings, depending on whether a wye or delta connection is used for the parallel configuration. This higher voltage, equivalent to a 6.2 kV line-to-line voltage, requires less current for the same power, with the subsequent efficiency increase on electromagnetic devices.

However, the overall plant is likely to cost more. Table I shows an increase of 7% for the generator and it is estimated around of 3% for the rest power conversion system. That is because of the required insulation requirements have a great influence on elements such as cables, switchgears, cooling system, and other auxiliary devices considered on the series configuration.

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Fig. 11 Decision chart for parallel and series connections

B. Power quality and filters The passive components have a strong effect on the size,

efficiency and cost of the power conversion system. This is the reason why they represent an interesting topic for designers and manufacturers, especially when they are applied to the grid side. The grid filters reduce the current harmonics injected to the grid and allow the fulfillment with the grid code.

The line inductor has a special relevance due to the fact it has a strong influence on the system performance. The series configuration provides a higher number of voltage levels that ensures a good harmonic cancellation regardless of any mismatch on the line output inductors. Also, the purity of the machine winding currents is much higher.

The line inductors are of half the value if we compare them with parallel configuration. This is mainly due to the redundancy features imposed on the parallel connection, which require complying with power quality standards when only one PCL is in operation.

C. Redundancy The parallel connection of two conversion lines shows the

benefits offered by a redundant power conversion system. To illustrate this fact, the total cumulative energy delivered over a year has been calculated assuming an unfortunate fault in one conversion line, straight away after the maintenance period. It is usual in offshore farms to have a one-year maintenance window. This calculation, shown on Table II, is based on typical figures of wind probability and power delivered for an offshore location. These indicate that the peak wind probability of 4.5% is reached at about an average wind speed of 6.5m/s, where the output power has a value of 0.53 p.u. over the rated power. Full power is reached at wind speeds above 8 m/s, with probabilities ranging between 3.6 % and 4 %.

TABLE II -CUMULATIVE ENERGY DELIVERED OVER A YEAR

Full system Redundancy Cumulative (GWh) 37.5 21.9 Percentage 100% 58%

It is clear by looking at the table above how the total energy delivered is higher than the percentage of conversion lines that are operating. For instance, a fault in one of the two available conversion lines would manage to deliver 58 % of the total available energy, higher than the 50 % (1/2) anticipated. This is explained by the fact that the maximum wind power is not always available due to low wind speed. Moreover, operating with less conversion lines at lower wind speeds may also increase the overall system efficiency. On the other hand, having a fault on a single PCL system will stop all power delivery.

D. Rated Power and Efficiency As already explained, a complete PCL involves 6 Phase

PEEB and 1 Chopper PEBB, all integrated in a single electrical cubicle. It has a rated power density of 0.45m3 /MW and a typical switching frequency of 1 KHz. The converter size and

overall joint machine and generator efficiency at 6 MW are presented in Table III.

With regards to the power conversion system efficiency, an estimated value of 97.6% and 97.7 % is obtained for the parallel and series configuration. The small difference between two solutions is mainly related to the reduction of the grid inductance in the series configuration. Taking into account the data shown in Table I, the overall machine-converter efficiency reached an estimated value of 96.13% and 96.23% respectively. Besides, the overall enclosure volume is estimated to be 18.4 m3 in both configurations.

TABLE III -OVERVIEW OF THE OVERALL EFFICIENCY AND CONVERTER SIZE USING PMG AND EESG MACHINES

Machine Type Joint system efficiency at rated power (%) Parallel / Series

Converter volume (m3)

PMG 96.13 / 96.23 18.4 18.4 EESG 96.13 / 96.23

E. Comparison chart The features described in the preceding sections have been

presented in a decision chart format to offer a more straightforward view. The decision chart for the converter solution is proposed on several key indicators for an offshore approach. These are: Output power; output voltage; volume; weight; power density; scalability options; grid-side filter, or its design requirements, volume and efficiency; efficiency; grid code and LVRT compliance and redundancy. The different options presented in Fig. 11 are benchmarked using a 0 to 5 score system, where 5 is given for the best option. The remaining option is then rated proportionally. That is, the further away from the centre is a curve, the better that option is. Therefore, if one of the proposed arrangements surrounds its counterpart in the chart, it would mean that it is superior.

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V. CONCLUSIONS In this paper, two power conversion topologies have been

studied to achieve a 6 MW wind turbine for off-shore applications. The first one is based on combine two conversion lines, electrically isolated by means of separate dc-buses and two sets of independent stator machine windings. The second one is achieved by combining two conversion lines in series, by means of having open-end terminal connections at the grid transformer and machine side.

The series configuration presents some important advantages with respect to parallel configuration such as higher voltage levels and smaller grid filters which makes the whole be slightly more efficient. However, the redundancy and scalability features which are inherent in the parallel configuration makes it very suitable for a wind-turbine off-shore application where a fault in the power conversion system could involve a significant fall in the yearly's energy production. Besides, the overall plant is likely to cost less than in series configuration.

The results presented in this paper show that the two configurations under discussion are suitable candidates, each having its own advantages and disadvantages. Therefore, both configurations should be analysed in more detail when looking at particular wind-turbine requirements.

ACKNOWLEDGMENT The authors would like to thank Jon Vaquerizo and Javier

Sanchez, from Indar, electric motors and generators manufacturer, part of the Ingeteam group, for providing the generator data.

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