Compensation of reactive power in electrical networks

Electrical equipment consumes energy during operation. In this case, the total power consists of two components: active and reactive. Reactive power does no useful work, but introduces additional losses into the circuit. Therefore, they strive to reduce it, for which they come to various technical solutions for compensating reactive power in electrical networks. In this article, we will look at what it is and what a compensating device is for.

Definition

The total electrical power consists of active and reactive energy:

S = Q + P

Here Q is reactive, P is active.

Reactive power occurs in magnetic and electric fields, which are characteristic of inductive and capacitive loads when working in AC circuits. When a resistive load is operating, the voltage and current phases are the same and coincide. When an inductive load is connected, the voltage lags behind the current, and when a capacitive load is connected, it leads.

The cosine of the shift angle between these phases is called the power factor.

cosФ = P / S

P = S * cosФ

The cosine of the angle is always less than one, so the active power is always less than the full one. The reactive current flows in the opposite direction relative to the active one and prevents its passage. Since the full load current flows through the wires:

S = U * I

Even when developing projects for power transmission lines, it is necessary to take into account the consumption of active and reactive energy. If there is too much of the latter, then it will be necessary to increase the cross-section of the lines, which leads to additional costs. Therefore, they are struggling with it. Reactive power compensation reduces the load on the grid and saves industrial energy.

Where it is important to consider the cosine phi

Let's figure out where and when reactive power compensation is needed. To do this, you need to analyze its sources.

An example of a basic reactive load is:

• electric motors, collector and asynchronous, especially if in operating mode its load is small for a particular motor;
• electromechanical actuators (solenoids, valves, electromagnets);
• electromagnetic switching devices;
• transformers, especially when idling.

The graph shows the change in cosФ of the electric motor when the load changes.

The basis of the electrical equipment of most industrial enterprises is the electric drive. Hence the high consumption of reactive power. Private consumers do not pay for its consumption, while businesses pay for it. This causes additional costs, from 10 to 30% or more of the total electricity bill.

Types of compensators and their principle of operation

In order to reduce the reagent, reactive power compensation devices are used, the so-called. UKRM. In practice, they are most often used as a power compensator:

• capacitor banks;
• synchronous motors.

Since the amount of reactive power can change over time, it means that compensators can be:

1. Unregulated - usually a capacitor bank without the possibility of disconnecting individual capacitors to change the capacity.
2. Automatic - compensation steps change depending on the state of the network.
3. Dynamic - compensate when the load quickly changes its character.

The circuit uses, depending on the amount of reactive energy, from one to a whole bank of capacitors that can be input and output from the circuit. Then the control can be:

• manual (automatic switches);
• semi-automatic (push-button posts with contactors);
• uncontrolled, then they are connected directly to the load, turn on and off with it.

Capacitor batteries can be installed both at substations and directly near consumers, then the device is connected to their cables or power buses. In the latter case, they are usually calculated for individual compensation for the reagent of a specific motor or other device - it is often found on equipment in 0.4 kV electrical networks.

Centralized compensation is performed either at the border of the balance section of the networks, or at the substation, and it can be performed in 110 kV high-voltage networks. The good thing is that it unloads high-voltage lines, but the bad thing is that the 0.4 kV lines and the transformer itself are not unloaded. This method is cheaper than the others. At the same time, it is possible to centrally unload the low side of 0.4 kV, then the UKRM is connected to the buses to which the secondary winding of the transformer is connected, and accordingly it is also unloaded.

There may also be a group compensation option. It is an intermediate type between centralized and individual.

Another way is compensation with synchronous motors, which can compensate for reactive power. It appears when the engine is in overexcitation mode. This solution is used in 6 kV and 10 kV networks, and also occurs up to 1000V. The advantage of this method over the installation of capacitor banks is the possibility of using a compensator to perform useful work (rotation of powerful compressors and pumps, for example).

The graph shows the U-shaped characteristic of a synchronous motor, which reflects the dependence of the stator current on the field current. Under it, you see what the cosine phi is. When it is greater than zero, the motor has a capacitive nature, and when the cosine is less than zero, the load is capacitive and compensates for the reactive power of the rest of the inductive consumers.

Conclusion

Let's summarize by listing the main theses on reactive energy compensation:

• Purpose - unloading power lines and electrical networks of enterprises. The device may include anti-resonant chokes to reduce the level harmonics in the network.
• Individuals do not pay bills for it, but businesses do.
• The compensator includes capacitor banks or synchronous machines are used for the same purposes.

We also recommend watching useful videos on the topic of the article:

Related materials:

• Causes of power loss over long distances
• How to determine the power consumption
• Wireless transmission of electricity over distances

According to the book V.E. Kitaev, L.S. Shlyapintokh "Electrical Engineering with the Basics of Industrial Electronics" paragraph No. 54 for the book issue 1968 and paragraph # 53 for the 1973 edition book, clearly states:... "that in an alternating current circuit containing only inductance, current lags behind voltage... and is ahead of the EMF of self-induction. We can say that in the inductive circuit, the voltage is ahead of the current in phase by 90 degrees.

As for the capacitive load in the same book (next paragraph # 55 issue of 1968. and # 54 for the 1973 issue) it says:... "when charging and discharging a capacitor... The current is ahead of the voltage in phase by a quarter of a period, i.e. 90 degrees ".

And you have written everything the other way around ...