Understanding the Components

Power Factor Correction - Understanding the Components

Inductors

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Purpose
The term "inductor" refer to a wide category of inductive components, with a big variety of applications and goals, but they all act like “energy tanks”. For example, by considering the output filtering stage of a high frequency DC/DC converter, the output inductor continuously swaps the electrical energy with the output capacitor. As the inductor function is to contain the variation of current, the current through the inductor is normally mainly direct current with a superimposed high frequency current ripple (at the switching frequency, or its double value, depending on the topology).

 Similarly, by considering the Power Factor Correction (PFC) stage of a high frequency converter, there is a high frequency energy swap involving the inductor and the capacitor; in that case, the current through the inductor is mainly composed by the mains frequency harmonic and a superimposed current ripple at the working frequency of the PFC stage. Similar inductors are also used in non-isolated high frequency conversion topologies (Buck, Boost, Buck-Boost), in which the current waveforms change depending on the mode of conduction, that is if the converter works in continuous or discontinuous mode.

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Features
Industrial applications often require dedicated inductors, as every one needs well defined characteristics. For this reason, Sirio chose to dedicate great efforts to customized choke development, automatic production-oriented, instead of an inductors catalogue, based on standard raw materials, typically available at our wharehouse. This kind of approach makes possible a just in time production of a custom component with the same cost of a catalogue product.
Main features of inductors are the following ones.

  • Ln rated inductance value
  • Rn rated winding resistance
  • In rated current (average value in case of output inductors)
  • Imax maximum current (the core has not to saturate at that peak current value)
  • Irms rms current (thermal current value)
  • ΔI peak-to-peak ripple current
  • fn rated working frequency (ripple frequency)
  • Un maximum working voltage (maximum voltage across the winding)
  • Up isolation voltage between winding and core or between winding and case

Inductors - Choice of Cores

The fundamental purpose of an inductors' magnetic core is to provide an easy path for flux in order to facilitate flux linkage, so that the magnetic energy can be stored in a non-magnetic low permeability region.
In inductors' development, the choice of the magnetic material is mainly related to the following parameters: working frequency, current waveform, maximum dimensions and environment conditions
As a matter of fact, the core selection is the first designing step and after that all other building details are defined: the number of turns and the conductor element (that can be copper foil, solid wire, litz wire), the isolation, the output connections.
Several cores are available on the market: they are different because of chemical composition, production process and dimensions. 
These elements define the main features of the core, like for example saturation flux, frequency behavior, temperature behavior.

                

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For high frequency applications, the most used materials are:

  • Soft Ferrite
  • Amorphous and Nanocrystalline Laminations
  • Powder (so called distributed airgap materials)

For lower working frequencies other materials are available:

  • FeSi Lamination
  • FeNi Lamination

By looking at the materials features, it is possible to make the choice of the magnetic material.

Soft Ferrite Cores
Soft ferrite is the cheapest and the most used material in high frequency applications. As a matter of fact, this term refers to a wide range of ceramic materials, which are obtained by sinterization of Iron Oxide and other metal oxides. Materials available on the market are usually identified with a code or a commercial name and they possess different characteristics (for example permeability, saturation flux, specific losses, temperature behavior, …). The specific material, the shape and dimensions define core performances and what kind of inductive component is suitable for.
In high frequency power conversion system ferrite materials have saturation flux in the range of 0,3¸0,5 Tesla and some thousands of relative permeability. Therefore, ferrite materials has quite low saturation flux, that is the most critical limit for inductive components, especially in case of high DC currents (like output inductors) or high overcurrent risk. Ferrite materials for power application have the great advantage of low losses level on a wide frequency range.

Amorphous and Nanocrystalline Laminated Cores
Amorphous and nanocrystalline laminations are obtained by a special rapid quenching technology. Typical thicknesses of those laminations are few tens of μm. Material features are defined by the chemical composition (materials are normally classified in Iron based and Cobalt based) and the production process (that in case of nanocrystalline materials involves the annealing phase too).
For few tens of working frequencies and critical environment conditions those materials have to be preferred because of high saturation flux (more than 0,9 Tesla), high permeability (in the 10000¸100000 range) and very stable performances with temperature, even if the cost is much higher.
Amorphous cores are suitable for output inductors, when high DC current, low ripple current, extremely compact dimensions. In fact, high saturation flux level is suitable for high currents and low ripple current doesn't cause critical core losses.
Nanocrystalline cores have vary low losses, even for high variation of magnetic flux, so they can be used for switching transformers too.

Powder Cores
Powder cores differ in chemical composition. Powder cores are pressed with an organic or inorganic binder, that is responsible for storage of energy. As the non-magnetic area is distributed in the whole core's volume and it is not concentrated on a small region, they are called distributed airgap cores. Because of this peculiarity, powder cores are suitable for high direct current inductors (output inductors) and flyback transformers in critical applications. One of the most interesting feature of this kind of cores is the smooth and sweet permeability versus direct current curve, that underlines the robustness of this materials against overcurrents.
By changing the magnetic to non-magnetic parts ratio, the “equivalent” relative permeability alters in the range of 10 to 600; some standard values are quite common on the market (for example: 26, 60, 90, 125, 147, 300, 550). For a particular core, the inductance factor (AL) will depend on material, shape and dimensions.
Each material, identified by the producer with a code or a commercial name, possesses its own characteristics in terms of permeability, saturation flux, specific losses. They are typically very stable with the temperature and they can be used in critical environment applications. Powder cores are more expensive than ferrite cores, but normally cheaper than amorphous and nanocrystalline ones.

 

Thyristor / Dynamic PFC

Fast reaction

EPCOS portfolio features all key components for dynamic power factor correction (PFC) systems. Wind power plants, in particular, stand to profit from the application of such systems, which feature fast reaction times and minimum maintenance requirements.

Conventional PFC systems consist of a controller to regulate the power factor, capacitors, capacitor contactors, fuses and a switching cabinet, in which all these components are assembled. The integration of reactors is also standard for such systems if there are harmonics.

In these conventional systems, the reset time of the capacitors is at least 60 seconds due to the necessary discharge processes according to IEC 60831. The relatively slow switching of the mechanical relays commonly used in standard PFC controllers also fails to satisfy the requirements of fast-changing loads that demand a reaction in the millisecond range. The same applies to the electromechanical capacitor contactors, since they are not designed for switching processes in the millisecond range.

Even when special capacitor contactors are used, the capacitor is limited to about 5000 switching operations per year (one switching operation about every 105 minutes). At approximately 100,000 switching operations the lifetime of the contactor is possibly to low. Moreover, Fig. 1 shows that when standard contactors are used, the inrush currents are more than 150 times greater than the rated capacitor currents, which leads to considerable stress on the capacitor and consequently shortens its operating life. Even when recommended capacitor contactors with integrated charging resistors are used, inrush currents of up to 15 times the rated capacitor currents can occur.

FIGURE 1: INRUSH CURRENT AT THE CAPACITOR

The inrush current at PFC capacitors can be over 150 times greater than the rated current.

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A suitable fast controller with transistor outputs is needed to implement a dynamic PFC system, with the thyristor as the key component. In combination with the controller, thyristor modules operate with reaction times of less than 20 ms all the way down to 5 ms. These short reaction times are indispensable for compensating fast load changes. But thyristors also switch at the zero crossing of the current, thus avoiding the high currents and high stressing of the capacitors. Fig. 2 shows the oscilloscope image of the switching behavior of the TSM-HV50 thyristor module. In the TSM series, this switching at the zero crossing of the current is achieved by keeping the capacitors at the maximum supply voltage after turn-off. When the trigger signal for the switching process is generated, the thyristor does not switch until the supply voltage reaches this maximum, in order to switch the capacitor at the same voltage level.

FIGURE 2: SWITCHING CAPACITORS ON VIA THYRISTORS

The diagram shows a three-phase PFC system. When switching the capacitors on via thyristors, no surge currents occur.

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Modl, an electrical engineering company based in southern Germany (www.modl.de), has combined EPCOS components such as PFC controllers, thyristor switches and capacitors in the DynaWind 6000 system, which is designed specifically for wind power plants (Fig. 3). This PFC system is used in a wind turbine of the AN BONUS-AN68-1.3 MW type. Its key data:

  • 690 V/50 Hz on the low voltage side
  • 21 kV/50 Hz at the intermediate voltage level
  • 110 kV/50 Hz for the high voltage network

The PFC system employed here makes an overall electric power of 400 kvar available, which is graduated in eight steps each of 50 kvar. The system is programmed so that the power factor is between 0.98 and 1, if possible. Semiconductor-based fuses are used to protect the system. Six vents on the roof of the switching cabinet as well as correspondingly large air-inlet slits with filters on the underside of the doors are used to cool the cabinet. No reactors were integrated in this first step.

FIGURE 3: DYNAWIND 6000 IN USE

The DynaWind 6000 consists of a BR6000T PFC controller combined with thyristor switches of the TSM-HV690 type, each of which can switch 50 kvar at 690 V. The capacitors used are of the WindCap series from EPCOS.

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The controller sends fast trigger signals to the thyristor switch and is equipped with an RS232 interface that allows a remote control and remote data readout.

Fig. 4 shows the measured power factor cos φ before and after turn-on of the dynamic PFC system in the model installation at the Oelerse Wind Park near Hanover. Whereas the power factor was previously in the 0.2 to 0.4 range, it subsequently improved to between 0.8 and 1.0.

FIGURE 4: POWER FACTOR COMPARISON

After putting the DynaWind 6000 into operation, the power factor rose to between 0.8 and 1

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The measured power factor fell below 0.98 on several occasions for two reasons: First, the smallest step width of 50 kvar did not always allow it to be fine-tuned. Because over-correction must be avoided, the attained power factor sometimes falls short of the specified target. Second, it also became evident that the installed correction power is under-dimensioned for the generator running under full load, even though the original installation was dimensioned for 400 kvar. No complete correction of the installation was consequently possible at full load. However, the full-load state occurred very rarely during the measurements.

Fig. 5 shows the reactive power occurring on the grid side. At the beginning and end of the measurement, the reactive power was in the range of -350 kvar with deactivated PFC. In this case, the negative sign indicates inductive reactive power. By switching on the PFC system, the reactive power dropped immediately to considerably lower values. It was already evident from the power factor measurement that the reactive power did not remain at a constant value of zero for the reasons already mentioned. The DynaWind 6000 system’s rapid reactions to load changes can be clearly seen in the measurement.

FIGURE 5: REACTIVE POWER ON THE GRID SIDE

Every step in this diagram represents a switching operation, normally 50 kvar

 

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Fast thyristor modules

EPCOS capacitors are specified for a maximum of 5000 switching processes per year if contactors are used. This number corresponds approximately to eight switching operations within the duration of 14 hours shown in the diagram (Fig. 5). It is readily apparent that wind powered installations require a considerably larger number of switching operations. In practice, up to 150,000 such operations per year are reached and the risk of the capacitors or electromechanical contactors failing with the associated consequences cannot be ignored. The thyristor switch solves this problem by switching the capacitors without stress. It thus enables not only fast switching, but also a nearly unlimited number of switching operations.

Optimal cooling

Cooling the installations is also of great importance. The electronic components employed such as the thyristors and cpaacitors, generate heat losses. Moreover, high temperatures occur inside the wind power turbines because of the exposition to direct sunlight. Especially in summer, mean values can reach more than 40°C and peak values in excess of 60°C in such systems. Standard MKK and MKP capacitors are defined in the highest temperature class for capacitors according to IEC 60831, -40/D, and are consequently designed for a mean continuous temperature of 35 °C. If these temperature limits are exceeded, their operating life is significantly shortened. As a rule of thumb, the life expectancy of a capacitor drops by a factor of 0.5 for every 7 K rise in temperature.


This problem can be solved by optimizing the cooling system or by using a different type of capacitor designed for higher operating temperatures, such as the MKV capacitors from EPCOS with a permissible continuous operating temperature of 45 °C or 70 °C for short intervals. Moroever, they also have a higher pulse strength of up to 300 times the rated current, allowing them to handle higher inrush current stresses than MKK or MKP capacitors. Thanks to these properties, MKV capacitors from EPCOS are also particularly well suited for conventional PFC systems with no protective switch-on via thyristors.

All capacitors types described are self-healing, i.e., in the event of a dielectric breakdown, the metalization vaporizes at the damage point so that no permanent short circuit occurs. In addition, they all have an internal overpressure fuse that triggers when the capacitor nears the end of its operating life and impermissibly high pressure builds up inside.

EPCOS offers all the key components for dynamic, conventional and static PFC systems.