What is reactive power? Reactive power compensation. Reactive power calculation

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

In apartments and private houses, one electric meter is installed, according to which payment for consumed energy is calculated. Simplistically, it is believed that only its active component is used in everyday life, although this is not entirely true. Modern housing is saturated with devices in the circuits of which there are elements that shift the phase. However, the reactive power consumed by household appliances is incomparably less than that of industrial enterprises, so it is traditionally neglected when calculating payments.

A plant 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. Atmospheric air around the power transmission line heats up completely uselessly; windings of transformers installed in substations may not withstand the load, especially during peak periods.

Load inductive and capacitive

If you take an ordinary heating device or an electric light bulb, then the power indicated inthe corresponding inscription on the flask or nameplate will correspond to the product of the values \u200b\u200bof the current passing 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 graph of the current flowing in them lags or leads the sinusoid of the supply voltage - in other words, a phase shift occurs. An ideal capacitive load shifts the vector by -90, and an inductive load by +90 degrees. Power in this case is the result of not only the product of current and voltage, a certain correction factor is added. Where does this lead?

Geometric reflection of the process

From the school geometry course, everyone knows that the hypotenuse is longer than any of the legs in a right triangle. If active, reactive and apparent power form its sides, then 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, subtracting, they are of different signs) the total vector, that is, the total reactive power, depending on which type of load prevails in the circuit, will be directed up or down. By its direction, one can judge which nature of the load prevails.

Reactive power with vector addition to the active component will give the total amount of power consumed. It is shown graphically asthe hypotenuse of the triangle of power. The more this line will be gently located in relation to the x-axis, the better.

Cosine phi

The graph shows that the angle φ is formed by two vectors, full and active power. The less their values differ, the better, but reactive power, which is considered parasitic, prevents their complete merging. The larger the angle, the higher the load on power lines, step-up and step-down transformers of the power supply system, and vice versa, the closer the vectors are inclined to each other, the less the wires will heat up throughout the circuit. Naturally, something had to be done about this problem. And the solution was found, simple and elegant. Mutual compensation of reactive power allows you to reduce the angle φ and bring its cosine (also called power factor) as close as possible to unity. To do this, it is necessary to lengthen the vector of the capacitive component so as to achieve a resonance of the currents, at which they "extinguish" each other (ideally completely, but in practice - to the greatest extent).

Theory and practice

All theoretical calculations are more valuable, the more applicable they are in practice. The picture in any developed industrial enterprise is as follows: 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: in real production conditions, reactive power of an inductive nature predominates. It should be noted that enterprisesthey install not one electric meter, as in houses and apartments, but two, one of which is active, and the other is easy to guess which one. And for the overexpenditure of energy "chased" in vain 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 one cannot do without electric capacitance when solving this problem.

Theory Compensation

From the above graph, it is quite clear how to achieve a reduction in parasitic currents up to their complete elimination, at least theoretically. To do this, a capacitor of the appropriate capacitance should be connected in parallel with the inductive load. The vectors, when added, will give zero, and only the useful active component will remain.

Calculation is made according to the formula:

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

It seems - what is easier? Multiply "X" and the number "pi" by 50 and divide. However, things are somewhat more complicated.

How is it in practice?

The formula is simple, but determining and calculating X is not so easy. To do this, you need to take all the data about the devices, find out their reactance, and in vector form, and even then … In fact, no one does this, except for students in laboratory work.

You can determine the reactive power in another way, using a special device - a phase meter indicating the cosine phi, or by comparing the readings of the wattmeter,ammeter and voltmeter.

The matter is complicated by the fact that in a real production process, the load is constantly changing, since some machines are turned on during operation, while others, on the contrary, are disconnected from the network, as required by the technological regulations. Accordingly, ongoing measures are needed to monitor the situation. Lighting works during night shifts, air can be heated in the workshops in winter, and air can be cooled in summer. One way or another, but reactive power compensation is based on theoretical calculations with a large share of practical measurements cos φ.

Connecting and disconnecting capacitors

The easiest and most obvious way to solve the problem is to put a special worker near the phase meter who would turn on or off the required number of capacitors, achieving the minimum deviation of the arrow from unity. So at first they did it, but practice has shown that the notorious human factor does not always allow achieving the desired effect. In any case, reactive power, which is most often inductive in nature, is compensated by connecting an electrical capacitance of the appropriate size, but it is better to do this automatically, otherwise a negligent worker can bring his own enterprise under a large fine. Again, this work cannot be called qualified, it is quite amenable to automation. The simplest scheme includes an optical electron pair of a light emitter and a light receiver. The arrow has covered the minimum value, which means you need to addcapacity.

Automation and intelligent algorithms

Currently, there are systems that allow you to reliably keep 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 automatic reactive power compensator still gives an economic effect very good. The operation of this device is based on intelligent algorithms that ensure operation immediately after switching on, most often even without additional settings. Technological advances in computer technology make it possible to achieve uniform connection of all stages of capacitor banks in order to avoid premature failure of one or two of them. Response time is also minimized, and additional chokes reduce the amount of voltage drop during transients. A modern enterprise power control panel has an appropriate ergonomic layout that creates conditions for the operator to quickly assess the situation, and in the event of an accident or failure, he will receive an immediate alarm signal. The price of such a cabinet is considerable, but it is worth paying for it, it brings benefits.

Compensator device

A conventional reactive power compensator is a metal cabinet of standard dimensions with a control and management panel on the front panel, usually opened. At the bottom of it are sets of capacitors (batteries). Suchthe location is due to a simple consideration: the electrical capacities are quite heavy, and it is quite logical to strive to make the structure more stable. In the upper part, at the level of the operator's eyes, there are the necessary control devices, including a phase indicator, with which you can judge the magnitude of the power factor. There are also various indications, including emergency ones, controls (on and off, switching to manual mode, etc.). The evaluation of the comparison of readings of measuring sensors and the development of control actions (connecting capacitors of the required rating) are performed by a circuit based on a microprocessor. Actuators operate quickly and silently, they are usually built on powerful thyristors.

Approximate calculation of capacitor banks

In relatively small plants, the reactive power of a circuit can be roughly estimated by the number of connected devices, taking into account their phase-shifting characteristics. So, a conventional asynchronous electric motor (the main "hard worker" of factories and plants), with a load equal to half of its rated 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 cosine φ equal to 0.8. The tables available to almost every chief power engineer contain information on almost all types of industrial equipment, and pre-setting reactive power compensation can be done using them. However, such dataserve only as a baseline on which to make adjustments by adding or removing capacitor banks.

Nationwide

You might get the impression that the state has entrusted factories, factories and other industrial enterprises with all the concern about the parameters of the power grid and the uniformity of the load on it. This is not true. The power system of the country controls the phase shift on a national and regional scale, right at the exit of its special product from power plants. Another issue is that the compensation of the reactive component is carried out not by connecting capacitor banks, but by a different method. To ensure the quality of energy supplied to consumers in the rotor windings, the bias current is regulated, which is not a big problem in synchronous generators.