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Posted on 01 December 2019

Power Factor Correction

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Dynamic progress

A change in technology is taking place in power factor correction. Static PFC systems are being progressively replaced by dynamic systems that offer new technical advantages and economic benefits.

By Rudolf Müller, Dipl.-Ing., Product Manager, PFC Power Capacitors, EPCOS.

 

Conventional power factor correction systems consist of a power factor controller and power capacitors. They are connected to the power line via electromechanical capacitor switching contactors. However, due to the discharge time of the capacitors, resetting times are longer than 60 seconds.

New applications are increasingly calling for technologies that respond in real time. Here dynamic PFC systems such as electronic thyristor modules are replacing slow electromechanical switches. As well as shorter response times, longer service life is an important advantage of dynamic systems, because thyristors are not prone to mechanical wear. Dispensing with mechanical capacitor contactors eliminates a further problem: high inrush currents. The thyristor modules switch the capacitors at the zero crossing of the current, thus avoiding inrush currents that can be as high as 200 times the rated current.

Dynamic systems open up new applications and offer a host of advantages:

  • Reduction of reactive power and lower energy costs in power distribution systems, even with rapidly changing loads
  • Switching times shorter than 20 ms
  • Reduced capital expenditure for new plant (power distribution systems, transmission systems, cable cross-sections, etc.) thanks to avoidance of peak currents
  • Stabilization of line voltage, e.g. no voltage dip during welding
  • Prevention of flicker
  • Smooth, transient-free switching
  • Longer service life of PFC system and equipment connected

Dynamic power factor correction is used in welding equipment, industrial presses in the automotive industry, wind parks, cranes, lifts, and main motor starting compensation, where it makes the soft starter redundant.

When a low-voltage power capacitor is connected straight to a power line without damping, the effect on the capacitor is similar to that of a short circuit. Capacitors connected in parallel and charged capacitors in particular cause extremely high inrush currents. To avoid adverse effects on power quality and capacitor service life, the inrush currents must be adequately damped.

An inrush current 157 times higher than the rated current is shown in Figure 1. This was caused by capacitors connected in parallel with a conventional motor contactor. The effect on the power line voltage is shown in Figure 2. Voltage transients that can have serious consequences such as insulation breakdown, damage to other loads or malfunctions in data systems and instruments. Although the extremely high inrush current can be avoided by using special capacitor switching contactors with leading and precharging resistors, a certain pulse current is inevitable (Figure 3).

Capacitor inrush current for contactor circuit

This problem can be remedied with thyristor modules that permit any number of switching cycles and offer short switching times for rapidly changing loads. As the capacitors are switched at current zero crossing by the thyristor, high inrush currents are avoided.

Voltage transients caused by current peaks

The thyristor switches the capacitor virtually without delay, as shown in Figure 4. As soon as the controller signal is applied to the thyristor, the current starts to flow through the capacitor and increases from zero to the peak value without any inrush current. As no inrush current peaks occur, no dangerous voltage transients are generated either.

Contactor configuration with and without damping resistors

The test curve shows how the initial sinusoidal current is distorted. This is caused by harmonics upstream. For this type of harmonic contamination, a dynamic power factor correction system with PFC capacitors combined with inductors should be used to avoid overloading the capacitors.

Capacitor current switched by thyristor

One typical application of dynamic PFC is found in the steel industry. Presses and welding equipment are operated in parallel with the power line. Fast switching times are inevitable due to the fast load changes. Thanks to several dynamic PFC systems, de facto real-time control has been achieved. The design of the dynamic system has resulted in significant reduction of reactive power. Capital expenditure for the low-voltage power supply (new busbar system, new transformer, low-voltage main distribution board, etc.) was thus reduced substantially.

Capacitor current after switch-on

Reduction of reactive power in the power supply is shown in Figure 6. The curves of the current with and without PFC are shown on the right and left respectively. The oscillogram Figure 5 shows that no inrush current peak occurs, but merely a slightly rising current without any harmful effect on power quality or the capacitors.

Current consumption with and without power factor correction

One-stop shopping for dynamic PFC

EPCOS offers a comprehensive range of products for dynamic power factor correction with two types of thyristor module. These include TSM-C modules that can handle reactive powers of 25 and 50 kVAr, the new dynamic power factor controller BR6000-T, PhaseCap PFC capacitors, and complete dynamic PFC systems. The self-monitoring TSM-C thyristor module is a dynamic electronic switch that can switch on PFC capacitors free of transients in a few milliseconds (Figure 7).

Switch-on graph of TSM-C module

It is distinguished by:

  • Ease of assembly: the thyristor module can be used like a capacitor switching contactor; complete intelligence is integrated into it
  • Self-monitoring for capacitors handling up to 50 kVAr
  • No harmonics generated because complete sine wave switched
  • Fast response times of less than 7 ms
  • Continuous self-monitoring of voltage, capacitor current and thyristor switch temperature
  • Cascading output
  • Alarm output on each module
  • Control and error message display

The dynamic PFC controller BR6000-T, Figure 8, is the result of ongoing development of the BR6000 series, to which new functions have been added. It was specially developed to control the thyristor modules for dynamic switching and corresponding power factor correction. A typical configuration with a BR6000-T is shown in Figure 9. In this example, the PFC controller drives a TSM-C thyristor module, which controls PFC capacitors combined with inductors. Thanks to the fast processor, short switching cycles are achieved. In addition to the switching time of less than 40 ms, the BR6000-T offers very fast setting of the power factor by simultaneous switching of several stages. Various parameters can be adjusted for perfect matching of the PFC controller to different thyristor modules.

Control panel of PFC BR6000-T from EPCOS

Another innovation makes it easy to couple two power factor controllers. For example, two power feeds can be supported with one coupling switch. This can also be done without a controller interface. The BR6000-T is available with six or 12 transistor outputs and one alarm terminal.

Circuit diagram of dynamic PFC system

A text-based, menudriven display makes the PFC controller very easy to use. The new features permit intuitive operation. Self-explanatory symbols and supporting text in the respective user language make handling simple.

Dynamic power factor correction in the steel industry

Summary

Dynamic power factor correction with thyristor modules offers significant advantages over static solutions with capacitor switching contactors:

  • Short switching times of less than 7 ms
  • No current peaks, therefore no dangerous transients and line pollution
  • Lower installation requirement for distribution at the low-voltage level
  • EPCOS offers one-stop shopping for dynamic power factor corrections:
  • Processor-controlled PFC controllers with up to 12 outputs
  • Fast-switching thyristor modules
  • PCB-free PFC capacitors with integrated overload protection
  • Inductors for damping PFC capacitors
  • Consultancy and installation service

 

 

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