Active power

Active power - is part of common consumed by the source. Came for the future load consumed. They write that the electrical energy required to turn in the other, this is not important. Reactive power is reflected back to the source. Others - the theme of today's discussion.

basic concepts

When the lesson of physics teacher tells about Ohm's law, it operates with the active components of current and voltage. Hence, their zero phase shift. And the power goes active. Calculated as the product of current and voltage. At a lesson of physics power is converted into heat on an abstract resistance. In life, as a rule, the negative effect of power loss on the wires. Useful are:

  1. current conversion in the motor rotor.
  2. Space heating.
  3. Illumination (lighting).
  4. Lighting the stove burners.
  5. Formation of the output standard voltage power supply.

Examples of mass. For instance, substation transformer load is considered to HPP. On power lines lose heat and the sound of the power is reflected. The latter is known as reactive, describes a chain reaction comprising the inductance (in case of a transformer), or containers, to an external impact. Some time elements of the power accumulated and then is given in the opposite direction. The question arises - why use such "damaging" reactive elements.

  1. Reactive elements transform the energies that are often required. For example, a transformer is used for galvanic isolation circuits of different voltage. Without coils gather it is not possible. Similarly, the need for filtering capacitors.
  2. The use of reactive elements are not always harmful. It is considered good form when an entity uses the reflected power of their own equipment. For exceeding the limit of an allowed level of reactive power is possible penalty for overloading of transmission lines and substation transformers. To avoid such, the inductive reactance motors equalize capacitive impedance of capacitor units. An oscillation circuit is formed, the reactive power circulates solely on circuits Company, causing considerable damage, for the most part, being deposited on the heat conducting.

Everything written above, gives the concept in the simplest form of the ongoing processes in the network. Students unable to explain the concept under consideration. Assume capacitor charging process. Voltage across it lags behind the current. whether reactive power? If after charging the capacitor is turned off, the plant is not fined. But power is still reactive - the current and voltage phase different:

P = IU cosφ, where φ - the phase angle between voltage and current.

What is the phase angle

Nikola Tesla saw the world as the air filled with vibrations of different frequencies. Matter of the harmonics generated. Tesla foretold, for example:

  • The advent of the Internet.
  • The central news broadcasts on radio and television.
  • Coverage of the planet's energy networks.

It now seems the world simple. Tesla envisioned a world after a hundred years. Fluctuation in physics and radio conveniently represented as a vector (directional length), rotating around the origin at a rate equal to the natural frequency. Is the angular frequency is ω = 2 pi f. This parameter is used in a number of formulas.

When the power source generates power, current and voltage are rotated synchronously with the zero phase shift. Of course, the reality is very different from the ideal, but it is clear what is happening. For voltage transformer secondary winding is written expression:

E2 = I2R2 + U2 + I2 2 Pi L, wherein:

  • I2 - secondary current, slightly lagging the voltage, but not by 90 degrees;
  • U2 - the output voltage to the coil, along with I2 is delivered to businesses and other consumers;
  • I2R2 - loss of heat in the ohmic resistance of the secondary winding (found by Ohm's law);
  • I2 2 pi L - reactive component voltage as seen from the figure, perpendicular to the current is delayed, becoming the cause of the presence of phase shift.

Thus, the inductive reactance leads to the fact that consumers are shipped substandard energy. To rectify the situation, put the substation capacitor banks. Then reactances cancel each other out, and the reactive power will circulate only through the territory of the substation. This is bad, but it's the principle of electromagnetic induction. Suppliers ship the net active power without phase shifts.

As mentioned above, the company will consume part of the power, but inevitably influence of parasitic effects. It is time to recall the definition given at the beginning. Certain sources claim that the active power is converted into other forms of energy. When setting the compensatory typed reactive power, then gives it to the inductance not indefinitely. Reactive power is dissipated as heat gradually the cables. Incorrect to speak about certain transformations. To summarize:

  1. In industry, the reactive power is called the energy delivered back through the supply chain. Effect from the beginning to the end of today is negative.
  2. In physics, there reactive power immediately when the phase shift occurs. Not always a parasitic effect.

Two definitions are closely linked, inseparably present in the literature. It remains to add that it is not always required to set compensatory installation at the substation. Resistance transmission line has a pronounced capacitive color. The negative effect is balanced by the skillful design. There is sometimes a need to install reactors to avoid a number of negative aspects.

Active power three-phase current

Active power three-phase network is the sum of each of the phases. The value is expressed in terms of linear variables. With symmetric consumption current through the neutral is not observed, the power is expressed by the relations represented in the image below. The formula is simple to understand. In a symmetric system the phases of the currents are equal, as the voltage directly summed. There is a factor of 3.

In turn, the line voltage at inclusion triangle constituting the usual case 380 more phase the root of three times. For currents there is no difference, they are equal phase. Scheme star equality causes the phase of the line voltage when the phase currents are greater. Therefore, in the last formula factor is the square root of three.

Experts will recognize that the star circuit operates at lower voltages, therefore the current consumption is reduced. But the question here is about the same for the derivation of power. In these conditions, if the voltage is decreased, the current increases. To calculate the reactive power represented by the expression must be multiplied by the sine of the angle and not the cosine. The total power is equal to the hypotenuse of the triangle bounded by the specified values. Calculated by multiplying the voltage and current at the root of the three without angle.


The above shows directly that in real systems, active power can not be separated from the rocket. Consistent with this is the set of applications described features. The first step is considered to be the introduction of individual values ​​to display two metrics:

  1. Active power is measured in watts. So taught in physics classes. Power shows generally counter mounted in the switch box in the stairwell.
  2. Full power is expressed in volt-amperes. This is a geometric sum of active and reactive component. Full power demonstrates, for which the company pays. The reflected energy does not bear good, purely economic loss.
  3. Reactive power is expressed in Vary. Sometimes the letters are written in capital, it turns out: kVAR, VAR, etc. Reactive power is measured by counters companies for different purposes: billing features supplier, setting the inductive reactance compensation system condenser equipment settings.

From the formulas given above, we conclude that the cosine of the angle shift voltage phases and current is numerically equal to the ratio of active power to complete, and the sine - reactive to complete.

power measurement

For each kind of power its own meter. We add that the physical principle used is the same, but different instrumentation device. For example, analog models operate on the principle that the open winter 1819-1820 gg. Hans Oersted. More precisely, the influence of the conductor on the compass arrow noticed before, but did not attract as much attention as has happened in the autumn of 1820. When the scientific world saw that electricity and magnetism are related.

So, at the heart of analog instrumentation lay mulplikator Johann Shveyggera (September 1820): the current passed through a coil of wire and deflects the arrow in the prescribed direction. Readings on the dial and entered manually in the table.

Modern devices work differently. The term meter is simplified to a single processor performing the discrete Fourier transform and calculating the required value. It is clear that it is important to find the phase shift and the current, the voltage is set a priori. The creators of the meters know that according to GOST voltage capable of walking by 10% in both directions. Consequently, one can not assume a priori specified voltage, the value also measured.

Then can only multiply on the formulas given above. In analog devices coefficients are given gear ratio mechanisms, the number of turns and so forth. In digital do without difficulty, in the presence of plenty of algorithms to calculate. I use the formula appeared much earlier than created the first computer. And the world was waiting for conformable application of computing power.

Analog power meter includes the main parts:

  1. Fixed-voltage coil. Oersted for it would sound strange, any coil creates a magnetic flux with a current. Voltage at anything. For measuring circuits carefully calculated coefficients, a high resistance is placed in parallel to the circuit (structurally included in wattmeter), which the current is limited. Do not stress! Small current controls the magnetic flux. Deflection is proportional to the voltage. This measuring principle is justified by Ohm's law for subcircuit.
  2. The fixed current coil incorporated directly into the circuit. Therefore, the minimum resistance expected. At high voltages, the measurement signal is taken transformer. Transfer factor is not its voltage is calculated, as it happens, and current. Knowing the coefficient of proportionality is easy to find the desired value. Consequently, the power meter is configured to use a transformer or a single value set a priori. Then the adjustment is not required, but you have to choose the transformer, the transmission coefficient of which corresponds to the requirements.

The movable frame arrow shows the result on the dial. Fixed coils are arranged in perpendicular planes. The frame is made of metal alloy, or taken inductor. Structure has been calculated so that the deflection becomes the desired proportionality factor and shows a sine phase angle (reactive power) or cosine (for the active power).



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