Power factor controllers for PFC systems
April 4, 2014
Everything under control
EPCOS power factor controllers allow complex PFC tasks to be carried out fully automatically. They also ensure that the capacitors switch with minimum wear and can measure a wide range of data at the same time.
Whenever inductive loads such as motors or transformers operate on AC power lines, a phase shift occurs between the voltage and current, which leads to the generation of reactive current and power. Reactive current is inevitably produced and thus incurs costs. Because it cannot be utilized, it should be avoided as far as possible.
Suitably rated capacitors connected in parallel to the inductive load can be used to correct, and thus avoid, reactive currents (Figure 1). In addition to saving costs for the consumer, the power grids are relieved and the power quality is improved.
The connection of inductors and capacitors in parallel creates a resonant circuit which corrects the phase angle between the voltage and current. In the ideal case, the phase angle is then 0° and the load absorbs only effective power.
Individual loads can be corrected directly in a very simple way, but are an exception. In real applications in the sectors of industrial electronics or building services, numerous individual loads with the most diverse switching cycles must be corrected. This can be done optimally and automatically only with power factor controllers (PF controllers). TDK offers a wide range of EPCOS PF controllers for this purpose. Thanks to their diverse performance features, they can efficiently solve all regular and many special correction tasks.
Correct system dimensioning
In a first step, the total reactive power of the system must be determined. It is the sum of all reactive power components which can occur simultaneously. In a second step, the number and size of the switching stages must be defined. These are essentially determined by the number and size of the loads. The more small loads that are present, the more stages will be required to achieve sufficiently accurate correction. The switching times must also be considered, as frequent switching subjects both the capacitors and contactors to high stress.
For smaller correction tasks, the EPCOS BR604 PF controller with four outputs is sufficient. The BR6000 series offers six or twelve outputs, whereas the BR7000 series has up to 15 outputs (Figure 2) – depending on the version.
All EPCOS PF controllers are microprocessor-controlled. They continuously measure the differential reactive power between the power factor of the line and the desired target value (cos Φ). The capacitance required to achieve this target is then switched-in automatically in stages on that basis. The controllers offer the following performance features:
- Intelligent control
- Automatic initialization for specific types
- Self-optimizing control
- Option for displaying maximum values
- Four-quadrant operation
- Wide voltage measurement range
- Alarm output
The key grid parameters can also be shown on the display. These include voltage, current, and frequency, as well as effective, apparent and reactive power, harmonics, cos Φ, and more. The display of the maximum values and the test run option allow simple fault analysis and system monitoring. All controller versions with interfaces are supplied with the corresponding software BR7000-SOFT for Windows. This allows the display and storage of all parameters on the PC, graphical analysis and evaluation of the data, storage and editing of the controller parameters via PC, as well as online operation of one or several PF controllers.
All PF controllers, aside from the BR604, offer a menu in ten language version (CZ/E/ES/F/GER/NL/PL/PT/RU/TR). Twelve controller variants with relay or transistor outputs are currently available.
Conventional or dynamic correction
In conventional operation, the capacitors are switched-in via capacitor contactors driven by the relay outputs of the PF controller. This switching mode subjects the contactor contacts to a certain wear, and also places the capacitors under considerable stress. This is particularly the case when the capacitors are not yet completely discharged and – in the worst case – are reconnected to the power line when out of phase. So a PFC system using capacitor contactors only makes sense where relatively few switching operations occur.
Industrial electronics applications often involve loads which are switched very frequently such as in welding equipment, presses, elevators, and various drive types. The TSM series of EPCOS thyristor modules for dynamic PFC is ideal for switching the capacitors in these applications. They should be used when more than 5000 switching cycles per year are expected. These modules are characterized by extremely short switching times of only 5 ms, zero wear, and silent switching. As the thyristor modules switch at the zero point of the current, the connected capacitors are protected from current surges, so that their operating life is significantly extended. Microprocessor control of the thyristor modules produces an automatic adaptation to tuned or detuned capacitor branches at a detuning factor of up to 14 percent. The modules offer automatic monitoring of the voltage, phase, and temperature. The status display takes place via LEDs. The EPCOS series of thyristor modules currently comprises eight types covering a voltage range from 230 V AC to 690 V AC and rated at between 10 kvar and 200 kvar.
The thyristor modules can be driven by all EPCOS PF controllers with transistor outputs. This includes the BR6000-T and BR7000-T PF controllers. The BR6000-T6R6 is a special case as it features six transistor outputs for dynamic and six relay outputs for conventional correction.
Capacitor turn-off at power failure
Brief power failures occur regularly in industrial networks. They are often not even noticed, as they last only milliseconds. A particularly critical case is when a fully charged capacitor is reconnected after a power failure when out of phase. It is then exposed to a voltage of 2 × VR × 1.41. This leads to an enormously high inrush current, which puts considerable stress on the capacitor. Figure 4 shows this case.
Inrush currents can even cause the contacts of the capacitor contactors to melt. To avoid such dangerous scenarios, all EPCOS PF controllers are equipped with a zero-voltage cut-off. In the event of a power failure, it disconnects the capacitors from the power line and only reconnects them after they have been discharged (40 s to 60 s).
The EPCOS capacitors offer the right solution to meet all the requirements of power factor correction. The Table shows the main PFC capacitors in overview.
|Series||Rated voltage [V AC]||Maximum capacitance [µF]|
|PhaseCap® Premium||230 to 800||3 x 251|
|PhaseCap® HD||400 to 525||3 x 332|
|PhiCap™||220 to 600||3 x 300|
|PoleCap||400 to 525||3 x 166|