Power factor#Power factor correction in non-linear loads

When the reactive power factor correction (PFC ), also called power factor correction reduces the undesired displacement power factor and the resultant reactive power electrical loads in AC systems. The reactive power compensation is usually done by the inductive reactive power compensation by means of capacitive loads.

Basics

Reactive power and the reactive power required for this purpose are needed for the generation of electrostatic or electromagnetic fields. Since continuously degrade these fields are created and again in the cycle of the AC voltage, the energy oscillates continuously between producers and electric consumers. This can not be used, or not be converted into another form of energy, but strain the electricity supply network and the production equipment (generators, transformers).

Too much reactive power is undesirable because the network is subjected to higher loads and larger ( synchronous ) generators are needed. It would further separate, magnetically excited synchronous generators are only required for reactive power compensation. Most generators are synchronous generators, the unnecessary reactive power as a " phase shifter " acting, to compensate, but the least electrical loads are provided with capacitive means, to compensate for the reactive power.

In larger systems, the sum of the mismatches can be large enough to burden the general power grid more than allowed. Usual energy meters measure active energy only. Preference will be given for large industrial customers special meter installed to measure the reactive energy in order to determine the resulting cost burden over time can.

The continuous cost justify a certain size, the installation of a capacitive load, the reactive power compensation system. This is joined to the central entry point to all inductive loads. The opposing, capacitive reactive power is ideally of the same magnitude as the installed inductive reactive power. This measure is called compensation.

The system consists of permanently installed or automatically switch capacitors ( active reactive power filter) which receive a capacitive reactive current, which is directed opposite to the usually inductive reactive power of consumers and picks it up by about 95%. If one were to fully compensate the inductive reactive power, the capacitive and inductive reactances would cancel and lose their effect, with the problem that the compensation due to fluctuations can then also move slightly towards capacitive overcompensation. Capacitive reactive power is connected on a larger scale with surges and regular technical difficulties, which is why it is avoided.

Theoretical considerations

In the technical field electrical consumables are usually resistive- inductive, that is, consumers need a magnetic field and relate inductive reactive power. A reactive power compensation requires the parallel connection of capacity, which in turn relate capacitive reactive power. A series circuit with the consumption of resources is not advisable because it would create a series resonant circuit, which results in a reactive current short circuit near its resonant frequency.

This is not true for devices with known data, where an exact compensation is possible (example: double switch with fluorescent tubes).

Reactive power in case of power without harmonics

The following discussion refers to sinusoidal voltage and consumables with sinusoidal and therefore harmonic- free power consumption. Due to the compensation system oscillates, the reactive current for the most part only between consumables and compensation system. The supply network is relieved.

The significant for the strain of the supply network size of the apparent power S is the geometrical sum of the active power P and reactive power Q. These are according to DIN 40110-1 as follows interrelated:

The total reactive power of the compensated load is made up of the inductive reactive power and capacitive reactive power.

The inductive reactance and the capacitive reactive power current are displaced 180 ° in phase, and therefore have opposite signs of the instantaneous values ​​. According to the stipulation that for inductive loads of the phase shift angle φ is positive, it also defines positive; conversely, φ and negative for capacitive loads. By correct in sign addition, the total reactive power is always less than any of the amounts of the individual reactive powers.

The reactive power component is typically a factor in the performance is the same in this case, of approximately

Compensated. In motor systems with asynchronous otherwise there is a risk of self-excitation, when the reactive power is fully compensated. In self-excitation of the motor with the disconnecting the power supply to the generator, and it can result in dangerous over-voltages. This case is referred to as resonance.

Another approach is based on the way that make complex load impedance is purely real by adding a reactance, so that is. On this condition, it is then also the dimensioning of the respective compensation component calculated in the form of a homogeneous equation.

Complete compensation is also often not feasible with simple capacitors or coils due to the swaying load a consumable. For that purpose, active power factor correction elements or so-called " network management systems " used at all times provide the required amount of reactive power is available.

For particularly large amounts of reactive power in power systems reactive power generators are used sporadically. These are synchronous generators, which can give inductive or capacitive reactive power depending on the agent state. They are also referred to as rotating phase shifter or a synchronous phase shifter.

Current state of the art, however, is the use of static VAR compensators. These are combinations of capacitances and inductances, which are arranged parallel to the load or to be compensated to the compensated network section. It is controlled by thyristor valves of the current flow in the individual components and thus the degree of reactive power compensation. Compared to the rotating phase shifter, this has the advantage that no deterioration of the system takes place, moreover, be a static compensator a much faster and more dynamic reaction to load variations possible.

Reactive power in case of power with harmonics

The above relations are valid only for sinusoidal path of voltages and currents, which generally is the case only for linear networks. In a circuit are nonlinear components such as magnetically saturating inductors or power supplies with rectifiers available, so the current is distorted, ie it contains harmonics. In addition to the reactive power Q of the fundamental oscillation occurs a distortion power on D, which summarizes the reactive power components of the harmonics.

The power factor correction means connected in parallel with the compensation filter, such as capacitors is only possible at a frequency, typically at the frequency of the fundamental component as the grid frequency. The reactive power of the remaining vibrations while compensation is too low or. Remedy provide this power factor correction, which are connected in series with the nonlinear consumer, and either dampen harmonics by appropriate filter structures, or artificially by a sinusoidal electronic circuits reproduce the fundamental frequency of the voltage corresponding to current flow in network side. This allows the power factor to a value close to 1.

• Current and voltage waveforms with harmonics

Compensated for by power factor correction to a power factor of 1

Example

The adjacent parallel circuit of a resistor and a capacitor is connected to the 230 V mains at 50 Hz, the currents specified. Due to the resistance to flow 2.3 A resistive current through the reactive current of 1.45 A has the connecting cable for the total current of 2.72 A be sized. For active power of 529 W is added a reactive power of 334 var, which states how much energy per time shuttling between the generator and condenser, lines and transformers unnecessarily burdened.

To compensate this reactive power a suitably chosen inductance of 0.5 H is connected in parallel to the device whose power factor is also 1.45 A. The reactive currents of the capacitor and coil compensate each other due to their opposite phase positions, and the total current consumption drops to 2.3 A. The parallel connection of coil and capacitor is ideally a parallel resonant circuit is that no reactive current from the generator absorbs at 50 Hz. Because P = Rline · I ² the power loss decreases in the supply lines to 100 % · ( 2.3 / 2.72 ) ² = 71 % of the original value.

Benefit

The reactive power compensation benefits the consumer, since he has to pay caused by the reactive current line losses which are dissipated as heat loss through the use of system charges ( § 10 Electricity Network Charges Ordinance). For the power company are reduced by the lower current the load of networks, and thus their (resistive ) lei ( s ) is lost by. To supply systems can be saved or must not be extended. To achieve this effect, the consumer, the reactive energy additionally measured at mass merchandisers ( special contract customers ) and calculated in the electricity bill. Therefore, there is a monetary incentive to keep the power factor within certain limits (for example ). Operates the consumer a functioning power factor correction system, so omitted in the ideal case all additional costs due to reactive power.

Audio frequency

When using the power factor correction in a network with ripple control system, the use of audio frequency may be necessary to prevent aspiration of the low-frequency ripple control signals from the network by the compensation system.

Others

The generator of today's wind turbines ( " wind turbines " ) is decoupled from the power supply through a DC intermediate circuit. Such a control of the phase shift between voltage and current in the phase current can be fed. These systems are no longer burden the network with reactive power, they are even used on the contrary for reactive power compensation.