What is reactive power? Reactive power compensation. Calculation of reactive power

In apartments and private houses there is one electricity meter, which calculates the payment for consumed energy. It is simplistic to assume that only its active component is used in everyday life, although this is not quite so. The modern dwelling is saturated with devices, in the schemes of which there are elements shifting the phase. However, the reactive power consumed by household appliances is incomparably less than that of industrial enterprises, therefore, when calculating payment, it is traditionally neglected.

A factory or factory whose management does not monitor the consumption of parasitic currents passing through the load circuit causes great harm to the energy systems of the region and the country as a whole. Absolutely useless heated atmospheric air around the power line; The windings of transformers installed in substations may not withstand the load, especially during peak periods.

Inductive and capacitive loads

If you take a conventional heating device or an electric bulb, the power indicated in the corresponding label on the bulb or the nameplate will correspond to the product of the current flowing through this device and the mains voltage (we have 220 volts). The situation changes if the device contains a transformer, other elements containing inductors, or capacitors. These parts have special properties, the current flowing in them lags behind or outstrips the sinusoid of the supply voltage - in other words, a phase shift occurs. The ideal capacitive load shifts the vector by -90, and the inductive one by +90 degrees. The power in this case becomes the result not only of the product of the current to the voltage, a certain correction factor is added. What does this lead to?

Geometrical reflection of the process

From the school course of geometry everyone knows that the hypotenuse is longer than any of the legs in a right-angled triangle. If the active, reactive and apparent power form its sides, the currents consumed by the coil and capacitance will be at right angles to the resistive component, but with directions in opposite directions. When adding (or, if you like, subtraction, they are multivalued) values, the total vector, that is, the total reactive power, depending on which load character predominates in the circuit, will be directed up or down. In its direction, one can judge which type of load prevails.

The reactive power in the vector addition with the active component will give the total amount of power consumption. It is graphically depicted as the hypotenuse of the power triangle. The more this line will be gently placed relative to the abscissa, the better.

Cosine fit

The graph shows that the angle φ forms two vectors, the total and active power. The smaller the differences, the better, but the reactive power, considered parasitic, interferes with their full merging. The larger the angle, the higher the load on the power lines, the raising and lowering transformers of the power supply system, and vice versa, the closer the vectors are to one another, the less the wires will warm all along the chain. Naturally, with this problem, something had to be done. And the solution was found, simple and elegant. The mutual compensation of reactive power makes it possible to reduce the angle φ and to maximize its cosine (which is also called the power factor) to unity. To do this, lengthen the vector of the capacitive component so as to achieve a resonance of the currents at which they "extinguish" each other (ideally ideally, and in practice, most).

Theory and practice

All theoretical calculations are of greater value, the more practical they are. The picture on any developed industrial enterprise is the following: most of the electricity is consumed by motors (synchronous, asynchronous, single-phase, three-phase) and other machines. But there are also transformers. The conclusion is simple: reactive power of inductive character predominates in real production conditions. It should be noted that at enterprises establish not one electricity meter, as in houses and apartments, but two, one of which is active, and the other - it's easy to guess which one. And for overspending in vain "driven" through power lines, the relevant authorities are mercilessly fined, so the administration is vitally interested in calculating reactive power and taking measures to reduce it. It is clear that without electric capacity, this task can not be avoided.

Compensation in theory

It is quite clear from the above graph how to reduce the parasitic currents until they are completely eliminated, at least theoretically. To do this, in parallel with the inductive load, turn on the capacitor of the corresponding capacitance value. Vectors at addition will give zero, and only useful active component remains.

The calculation is made by the formula:

• C = 1 / (2πFX), where X is the total reactance of all devices included in the network; F - frequency of supply voltage (we have - 50 Hz);

It seems to be - which is easier? Multiply "X" and the number "pi" by 50 and divide. However, everything is somewhat more complicated.

And how in practice?

The formula is simple, but it is not so easy to determine and calculate X. To do this, you need to take all the data about the devices, find out their reactance, and in vector form, and then ... In fact, nobody does this except students in laboratory work.

Determine the reactive power can also be different, using a special device - a phase meter indicating the cosine of phi, or by comparing the readings of a wattmeter, an ammeter and a voltmeter.

The situation is complicated by the fact that under the conditions of a real production process the amount of load is constantly changing, as some machines are switched on in the process of work, while others, on the contrary, are disconnected from the network, as required by the technological regulations. Accordingly, constant measures are needed to monitor the situation. During night shifts, lighting works, in winter, heating can be carried out in the shops, and in the summer it can be cooled. Anyway, but the reactive power compensation is based on theoretical calculations with a large share of practical measurements of cos φ.

Connecting and disconnecting capacitors

The simplest and most obvious way to solve the problem is to place a special worker near the phase meter that turns on or off the required number of capacitors, achieving the minimum deviation of the arrow from unity. So at first, and did, but practice showed that the notorious human factor does not always allow you to achieve the desired effect. In any case, the compensation of reactive power, which is often inductive in nature, is made by connecting an electrical capacity of the appropriate size, but it is better to do this automatically, otherwise a careless employee can bring his native enterprise to a large fine. Again, this work can not be qualified to be qualified, it can be fully automated. The simplest scheme involves an optical electron pair from the emitter and the light receiver. The arrow has closed the minimum value - means, it is necessary to add capacities.

Automation and intelligent algorithms

At present, there are systems that reliably hold cos φ in the range from 0.9 to 1. Since the connection of capacitors in them occurs discretely, it is impossible to achieve an ideal result, but the economic effect of the automatic reactive power compensator still gives very good results. At the heart of the work of this device lie intelligent algorithms that ensure the work immediately after turning on, often even without additional settings. Technological advances in the field of computer technology make it possible to achieve an even connection of all stages of capacitor banks in order to avoid premature failure of one or two of them. The response time is also minimized, and additional chokes reduce the amount of voltage drop during transients. The modern power management board of the enterprise has an appropriate ergonomic layout that creates the conditions for a rapid assessment by the operator of the situation, and in the event of an accident or failure it will receive an immediate alarm signal. The price of such a cabinet is considerable, but it costs to pay for it, it benefits.

Compensator device

A conventional reactive power compensator is a standard-sized metal cabinet with a control panel on the front panel, normally open. In the lower part of it there are sets of capacitors (batteries). This arrangement is due to a simple consideration: the electrical capacitances are quite heavy, and it is quite logical to strive to make the structure more stable. In the upper part, at the eye level of the operator, there are necessary control devices, including a phase indicator, by means of which it is possible to judge the value of the power factor. There is also a variety of displays, including emergency, controls (on and off, go to manual mode, etc.). Evaluation of the comparison of the readings of the measuring sensors and the generation of control actions (connection of the capacitors of the desired value) is performed by a circuit based on the microprocessor. The actuators work quickly and silently, they are usually built on powerful thyristors.

Approximate calculation of capacitor banks

In relatively small enterprises, the reactive power of the circuit can be estimated approximately by the number of connected devices taking into account their phase-shift characteristics. So, the usual asynchronous electric motor (the main "hard worker" of factories and plants) at a load equal to half its nominal power, has a cos φ equal to 0.73, and a fluorescent lamp - 0.5. The parameter of the contact welding machine ranges from 0.8 to 0.9, the arc furnace operates with a cosine of φ equal to 0.8. Tables available to virtually every major power engineer contain information on virtually all types of industrial equipment, and the pre-installation of reactive power compensation can be done with them. However, such data serve only as a basis, on the basis of which it is necessary to make adjustments by adding or removing capacitor batteries.

On a national scale

It may appear that the state has entrusted all the concerns about the parameters of the electric grids and the uniformity of the load to the factories, factories and other industrial enterprises. This is not true. The country's power system controls the phase shift on a national and regional scale, directly at the outlet of its special product from power plants. Another issue is that the reactive component is compensated not by the connection of capacitor banks, but by a different method. To ensure the quality of energy released to consumers in the rotor windings, the bias current is regulated, which in synchronous generators is not a big problem.