What is compensation in the power grid

Reactive power compensation

Lexicon> Letter B> Reactive Power Compensation

Definition: the reduction or targeted control of reactive power in a power grid

English: reactive current compensation

Category: electrical energy

Author: Dr. Rüdiger Paschotta

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Original creation: January 5th, 2013; last change: 19.09.2020

URL: https://www.energie-lexikon.info/blindleistungskompensation.html

In AC and three-phase networks there is the phenomenon of Reactive currents. (For a full explanation of the phenomenon, see the article on reactive current.) In some cases, attempts are made to avoid or compensate for this. One speaks of reactive power compensation as a rule only if this measure takes place near the place where the demand for reactive power arises. In this way, only a short line is loaded with reactive power.

A distinction is made between different types of reactive power, which are also different types of Power factor correction require; these are explained in the following sections.

Compensation of displacement reactive power

In the simpler case of Shift reactive power the electrical current strength oscillates due to inductive or capacitive loads with a certain phase shift (time shift) compared to the electrical voltage. This has a Reactive power The result: a power that oscillates quickly back and forth, which is 0 on average over time, i.e. loads the lines without transmitting energy on average. How strong this effect is is determined by the phase angle φ or, more frequently, by its cosine, the Displacement factor or Active factorcos φ specified; this is then also referred to as the power factor. The real power is P. = Ueff · I.eff · Cos φ.

Since inductive and capacitive loads cause phase shifts in opposite directions, the inductive and capacitive contributions to the reactive power can completely or partially offset each other. For example, you can compensate for inductive reactive power by additionally connecting a suitably dimensioned capacitive load (typically a capacitor) to the line. Conversely, a capacitive load can be caused by a Cross compensationthrottle be balanced. With the same strength of the inductive and capacitive reactive currents, a perfect reactive power compensation results (also as Power factor correction designated), d. H. there is no more reactive power in the common supply line, and there is φ = 0 and cos φ = 1. (The combination of inductive and capacitive load can then also be viewed as a parallel resonant circuit that has a very high impedance in resonance, i.e. takes in hardly any current from the outside.) The aforementioned oscillation of the current power only takes place between the consumer and the nearby compensation device, so it does not burden the common supply line.

In some cases it is also possible to avoid the development of reactive power from the outset, for example through a suitable selection of operating resources - for example, synchronous motors instead of asynchronous motors.

Compensation of distortion reactive power

Distortion reactive power is better avoided at the source than compensated for elsewhere.

Cases with a are more mentally complex Distortion reactive power; here there is a non-sinusoidal course of the current intensity. In practice, a compensation does not usually take place by including a compensation device that works completely independently of the consumer, but by appropriately modifying the consumer in order to avoid the distortion of the current flow from the outset. This of course means that such facilities must be individually tailored to certain types of consumers.

In particular, there is such technology for devices with a rectifier, which has to generate a direct voltage from the alternating voltage of the power grid. A simple built rectifier in connection with a capacitor to smooth the generated DC voltage generates strong distortions of the current (i.e. strong harmonics) because the current can only flow within an oscillation when the voltage is close to its peak value (namely above the capacitor voltage) . There are, however, more sophisticated circuits which, with the help of an additional switching device and a choke coil, generate a direct voltage at a significantly higher level and draw a current from the network, the instantaneous value of which is always roughly proportional to that of the voltage. Higher-frequency current components are also generated here, but they can be suppressed well with an additional filter. Such a technology is usually necessary for higher power rectifiers.

Meaning of reactive power compensation

The classic case of compensation for displacement reactive power is particularly common. Above all, inductive loads are often compensated for by adding capacitors. For example, companies that operate powerful electric motors can reduce their reactive power consumption. Conversely, however, capacitive loads (e.g. lightly loaded high-voltage lines) can also be compensated for by means of compensation chokes. Often it is only compensated to the extent that z. B. an effective factor cos φ of at least 0.9 is achieved.

The reactive power compensation serves on the one hand to reduce the current load on power lines, which on the one hand reduces energy losses and on the other hand keeps the line capacity free for the actual purpose of transmitting active power. In addition, reactive power compensation can also be used to maintain the voltage, since reactive currents also influence the voltage drops. However, reactive power is sometimes used specifically to influence the voltage, i. H. in such cases the total reactive power is not necessarily minimized.

In the case of uncompensated high voltage lines that are not or only slightly loaded, Ferranti effect occur: The voltage in the line is significantly higher than the applied voltage. This can lead to the destruction of equipment. This can be avoided by using suitable cross compensation chokes.

Basically, it must be weighed up whether reactive power compensation is worthwhile or whether it makes more sense to simply generate more reactive power in the power plants (which, however, puts more strain on the lines and generators). In the case of small consumers, no compensation is generally required, but compensation can be worthwhile for larger loads. From an economic point of view, reactive power plays a role for companies with high electricity consumption, as it is also recorded and calculated, which creates a financial incentive for reactive power compensation. A suitable structure of electricity tariffs can lead to macroeconomically optimized solutions.

A controllable (changeable) reactive power compensation is also possible with different methods:

• Individual capacitors or inductors can be switched on via thyristors, for example.
• With the help of a phase control, via which a choke coil is fed, a variable reactive power can also be generated. However, this also creates harmonics that might have to be filtered out with additional means. Therefore this method can only be used to a limited extent.
• Rotating reactive power compensators are based on an idling synchronous motor, the excitation of which is controlled in a suitable manner. This method is also suitable for very high reactive powers, but also leads to higher energy losses.

Synchronous generators in power plants can generate variable reactive power. This procedure is very widespread, but is usually not considered to be reactive power compensation in the true sense, since the required reactive power is not generated near the place where it is needed.

Side effects of reactive power compensation

An undesirable consequence of reactive current compensation with compensation capacitors can be a strong weakening of signals for ripple control. Such capacitors have a very low impedance for them. This problem can be avoided, however, by installing additional small chokes, which greatly increase the impedance at the frequencies relevant for ripple control, but have hardly any influence on the mains frequency.

Of course, devices for reactive power compensation should only be active as long as the loads to be compensated are in operation. Otherwise they can generate unwanted reactive power.