March 2011

Power capacitors for traction converters

On the fast track

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TDK-EPC is supplying power capacitors from EPCOS for the Velaro high-speed trains that Siemens is building for the Chinese state railway. They will be used in the resonant circuits of the traction converters.

China is expanding its railroad network according to a consistent schedule. The Velaro CN from Siemens, the fastest passenger trains so far operated by state railroad China Railways (CR) went into service in good time for the 2008 Olympic Games. This fourth series of high-speed trains is known in China as the CRH3 and is based on the German ICE 3. TDK-EPC has supplied EPCOS resonant circuit capacitors for a total of over 1000 traction converters.

In 2005, Siemens received an order for over sixty high-speed trains of the Velaro CN series from China Railways. They have wider railcar bodies than the ICE 3 and can accommodate six passengers on each row of seats. Overall, the 200 meter long Velaro CN offers more than 600 seats in its eight cars. The train weighs around 447 tons, is designed to run at a speed of 300 km/h and can be retrofitted to run at 350 km/h. The first five trains were already in operation in time for the 2008 Olympic Games (Figure 1).

Figure 1: The multiple unit principle of the Velaro

All 60 trains are now running a scheduled service on the 115 km route between the capital Beijing and the south eastern port city of Tianjin and take only 30 minutes for the trip. Apart from the first three trains, the Tangshan Locomotive & Rolling Stock Works (TLRW) in Tianjin is building the Chinese version of the Velaro CN.

Fifteen percent more seating space thanks to a new drive concept

The drive systems of the Velaro CN are not located in the power cars at the front and back of the train, but are distributed under the floor along the whole train (Figure 1). Sixteen of the 32 axles are motor-driven; each of the train’s four traction converters supplies four motors. This drive concept allows the rail operator to increase the seating space by almost another 15 percent. As a particular attraction, passengers have the opportunity to take a glance at the railroad track. However, it’s packed full under the train: apart from traction converters and drives, room must also be found there for transformers, air-conditioning equipment, compressed air generators, tanks for toilets and many other units. So relatively little space is available for the traction converters.

Resonant circuit needed for electric trains

Unlike the three-phase drives in industry, the traction motors of locomotives and trainsets run on a single-phase AC voltage supply. The overhead traction line voltage of the Velaro CN of 25 kV at 50 Hz is initially stepped down and rectified and finally smoothed with the aid of DC link capacitors.

Due to the single-phase supply, however, this DC link voltage is still superposed by a considerable ripple voltage of double the supply frequency. Additional filtering and interference suppression are performed by an resonant circuit, i.e. a series oscillating circuit comprising a capacitor and choke that is tuned to double the supply frequency.

The resonant circuit of the Velaro CN was implemented by parallel connection of two fixed capacitors as well as a tuning capacitor that combines four capacitors in a single case. This design also allows the resonant circuit frequency to be correctly adjusted after capacitance losses due to aging or divergences in choke inductance.

Customer-oriented solution

A basic design was developed from the Siemens specifications and simulation models were subsequently used to optimize it with respect to temperature profile, current and frequency response as well as the expected operational cycles of a high-speed train. As a solution for the resonant circuit, it was proposed to use MKK DCI power capacitors from EPCOS, i.e. oil-impregnated capacitors designed for operation with DC voltage (Figure 2). These capacitors additionally satisfy the high requirements on limit loads and fire prevention.

Figure 2: Resonant circuit capacitor

MKK DCI power capacitors are oil-impregnated and designed for operation with AC voltages. They additionally satisfy the high requirements on limit loads and fire prevention.

Each of the two fixed capacitors of the resonant circuits in the traction converters of the Velaro CN trains has a capacitance of 1520 µF and is designed for an operating voltage of 4135 V. The tuning capacitor has several tapping points and thus offers four capacitors in a case whose capacitance also totals 1520 µF (Table).

Table: Technical data of the EPCOS power capacitors

ParameterFixed capacitorTuning capacitor
Capacitance [µF]1520∑ 1520
Operating voltage4135 V DC4135 V DC
Peak voltage6000 V DC6000 V DC
Operating current [A]265265
Peak current [kA]175175
Operating temperature [°C]−50 to +70−50 to +70
Air humidity< 95%< 95%
Climate category50 / 60 / 5650 / 60 / 56
Fire load [g]< 300 / 1.74 MJ
Requirement 0, NFF 16 - 102
> 300 /1.74 MJ
Requirement 0, NFF 16 - 102
Dimensions (W × D × H) [mm]450 x 165 x 580450 x 165 x 580
Weight [kg]5657

Long operating life thanks to self-healing

The interior of the power capacitors comprises a structured film subdivided into individual segments. The resonant circuit capacitors of the Velaro CN thus consist of around 10,000 individual capacitors connected in parallel and series: they are stacked and contacted directly via the front face of the film (Figure 3).

Figure 3: Section through an EPCOS power capacitor

The interior consists of stacks of about 10,000 individual capacitors. By connecting them suitably in parallel and series, large capacitance values can be attained at high voltages.

This structural principle permits self-healing of the power capacitors and non-critical behavior in the event of overload: if a breakdown occurs in an individual capacitor, the metallization vaporizes on the spot until the spacing between the remaining metal surfaces becomes large enough again. If the current still continues to rise, the film melts away completely. The individual capacitor becomes highly resistive, averting the danger of further heating and thus a breakdown at other points of the capacitor.

The individual capacitors are connected directly to the terminal leads; the MKK DCI power capacitors consequently have not only a very low parasitic inductance but in particular a low internal resistance. Because very little wiring is required, these capacitors also have a low ESR value. As a direct consequence, the resonant circuit capacitors of the Velaro CN are distinguished by low self-heating and a very high pulse withstand capability of up to 175 kA. These properties are important because IGBT power switches are operated at ever higher switching frequencies.

Compact and reliable

The EPCOS resonant circuit capacitors are filled with rapeseed oil, giving the capacitor superior heat distribution properties than are possible with a dry casting material. In conjunction with oil impregnation, the capacitor film can withstand field strengths of up to 240 V/µm and thus permits a high energy density in the capacitor, permitting a highly compact case size of 450 x 165 mm x 580 mm3. The capacitor, which is welded into a stainless steel case, can nevertheless withstand voltage peaks of up to 6000 V AC.

These capacitors are extremely reliable, not least thanks to their optimized layout. They have an operating life of 200,000 hours, corresponding to continuous operation of about 23 years, and thus significantly exceeding the requirements of EN 50155. In addition, the MKK DCI capacitors have an extremely constant capacitance: under normal operating conditions, it declines by much less than 3 percent over their entire operating life.

Despite the continuous shocks and vibrations of the train, the electrical properties of the MKK DCI capacitors remain constant over their whole temperature range from −50 to +70 °C. The capacitors are qualified to IEC 1071 for the requirements of railroad operation.

IRIS-certified plants

As soon as the design of the resonant circuit capacitors was completed, samples were produced in the TDK-EPC plant in Malaga, Spain, some of which were used for operating life tests as well as for the electrical and thermal type approvals. Siemens incorporated the other capacitors in the first samples of the traction converters which were subsequently examined and tested according to the specifications of China Railways. As one of the first factories worldwide, TDK-EPC’s plant in Malaga was certified to the ISO 9001-based IRIS standards of the Association of European Railway Industries. The plant in Nashik, India, also recently received its IRIS certification.

In view of the market development in the sector of high-speed trains and freight locomotives, additional manufacturing capacity for power capacitors is currently being set up in India and China. These plants will not merely satisfy the local content of orders from the state railroads that is often required but above all allow the fast and flexible fulfillment of customer requirements.

In addition to resonant circuit capacitors, TDK-EPC supplies numerous other capacitors for the traction converters of the Velaro CN trains, such as for the filters or the circuitry of the IGBT power switches.

Development trends in power capacitors

In principle, the resonant circuit capacitors for the Velaro CN are suited not only for high-speed trains but – if only thanks to their long operating life – for all locomotives and trainsets. In addition, the use of tunable resonant circuits is ideal for operating electrical railcars in cross-border traffic (e.g. 16 2/3 Hz in central Europe and parts of Scandinavia, 50 Hz in France).

The design of the MKK DC/DCI capacitors allows their external dimensions to be dovetailed specifically to the relevant application so that the circuit layout in the converter can be simplified and optimized. This may be illustrated by the rectangular film capacitors of low insertion height whose footprint corresponds exactly to the dimensions of an IGBT power switch. This saves not only space but above all connection leads in the converter, leading ultimately to lower costs and higher reliability of the overall system.

In March 2009, China Railways ordered another 100 high-speed trains based on the CRH3. With their 16 cars, these new trains are twice as long as the previous models. At a speed of 350 km/h, the travel time for the approximately 1300 kilometers from Beijing to Shanghai is reduced to about 4 hours. The new CRH3 trains are also being equipped with resonant circuit capacitors from TDK-EPC.



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