November 2012

Aluminum electrolytic capacitors

Capacitance for gearless wind turbines

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A new generation of gearless full-converter wind turbines is entering the market. They need capacitors that can deliver high levels of capacitance, reliability and ruggedness. EPCOS screw terminal aluminum electrolytic capacitors provide an outstanding solution.

Reliability and ruggedness of all components are especially important for the fast-growing number of offshore installations. Also, with the increasing size of the rotor, especially in offshore installations, doubling the power (Figure 1) increases the torque on the gearbox 8-fold, escalating the cost of the gearbox itself and its associated maintenance costs. In response, wind turbine manufacturers have introduced gearless wind power plants. The generators are synchronous with voltages of 690 V AC (approx. 1200 V after 6- or 12-bridge rectifying). As a result, the entire power has to be handled by the converter, requiring very high capacitance levels per MW of power in the DC link in order to achieve the desired stabilization.

Figure 1: Increasing capacity of wind turbines

The use of ever larger rotors has led to a doubling of the turbine output power over the past 10 years. This, however, increases the torque on the gearbox by a factor of 8. In response, wind turbine manufacturers have introduced gearless wind turbines, which are more reliable and emit lower audible noise (important in populated areas). Above all, costly downtimes are reduced significantly.


Gearless and free of wear and tear
The new generation of wind turbines based on full-conversion topologies (Figure 2) eliminates the mechanical gearbox, which is subject to mechanical degradation and wear. They are therefore able to achieve these higher performance and reliability objectives. In order to be cost-effective in operation over the lifetime of the installation – as a rule more than 20 years under typical operating conditions – it is of vital importance that these components within the turbines remain maintenance-free.

EPCOS aluminum electrolytic capacitors with screw terminals represent an excellent solution. They meet the demanding requirements for capacitance as well as for reliability, long life, and temperature. The capacitors feature the high energy density required in confined wind power environments. These highly robust screw terminal components offer the performance needed in state-of-the-art direct-drive converters for wind power generators. The capacitors are connected in series and parallel to suit applications in wind power installations.

Figure 2: Wind turbine topologies
In the variable speed generators shown it can be seen that the direct-drive, gearless synchronous generator concept with full-scale converter allows for significantly smaller and lighter designs to be developed with higher output level and greatly reduced maintenance costs.

Tough requirements
By eliminating the gearbox, direct-drive generators require full-converter concepts which must comply with much stricter grid codes such as LVRT (low voltage ride through) and FRT (fault ride through). Other vitally important design factors to consider are the grid connection codes (Figure 3). Connection to the power grid needs to be clean in terms of reactive components plus the fact that the wind installation usually needs to remain connected or to compensate, even in the event of a system failure, in order to avoid the unbalancing effect on the grid which could cause severe disturbance or unwelcome outages in the supply.

Figure 3: Measured voltage breakdown response

Wind converters have to stay on the grid even in case of breakdown to 15 percent of rated voltage.

Wind generators especially may be required to be capable of compliance when the voltage in the grid is temporarily reduced due to a fault or load change in the grid. The voltage may be reduced in one, two or all the three phases of the AC grid. The severity of the voltage drop is defined by the voltage level during this condition (it may go down to zero) and the duration of the condition. Depending on the application, the system may be required to disconnect temporarily from the grid, but reconnect and continue operation after the fault condition, or to stay operational and not disconnect from the grid, or to stay connected and support the grid with reactive power.

The required LVRT behavior for generating units such as wind turbines (and solar power stations) is defined in grid codes issued by the grid operator. Examples of such grid codes are the E.On Grid Code in Germany or the National Grid Code in the UK. For critical loads such as power supplies for computer centers and industrial processes, the required LVRT performance is defined by the application. Because of this, converters in direct-drive generators require significantly higher capacitance to handle the higher power levels and to control the power factor. It is therefore vital that the choice of these capacitors will exceed the reliability of the conventional maintenance-intensive mechanical gearbox.

Portfolio for demanding applications
TDK is a reliable partner for highly demanding and environmentally rugged wind energy applications (and other renewable energy applications) with its extensive range of EPCOS aluminum electrolytic capacitors (Table). These compact capacitors, in screw-terminal versions, meet the designer’s needs for any converter specification, such as very long operating lives with maximum reliability under high AC current loads. Additional advantages of these capacitors are their low flammability and self-extinguishing electrolytes.

With the EPCOS aluminum electrolytic capacitors TDK is the world's leading manufacturer of DC link solutions for wind turbines and solar power generators. With its broad portfolio TDK can fulfill all converter specifications and provide the necessary customized engineering support.

Table: Aluminum electrolytic capacitors for wind power applications

SeriesMaximum operating
temperature
[°C]
Useful life
[h]
Rated voltage
[V DC]
Rated capacitance
[µF]

B43455 / B43457

85

10 000

350 to 450

1000 to 15 000

B43456 / B43458

85

12 000

350 to 550

560 to 18 000

B43564 / B43584

85

15 000

350 to 500

820 to 33 000

B43740 / B43760

105

6000

350 to 500

1000 to 18 000

B43700 / B43720

85

6000

600

1200 to 6800

Features

  • High ripple current capability
  • Very low ESR
  • Very low ESL (13 nH for low inductive designs)
  • Long useful life of up to 20 years

  • Compact can size

  • Optimized thermal concept

  • Terminals with protection against polarity (PAPR) on request

  • Special design for base cooling

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