Active, reactive and apparent power
Active, reactive and apparent powerDate of publication: 10-03-2023 Update date: 07-06-2024 🕒 4 min read
- The basic concepts of electrical engineering
- Active, reactive and apparent power
- Reactive power compensation
The basic concepts of power engineering
Electrical engineering is a vital branch of physics that is directly related to generating, processing, transmitting and utilising electric power. It describes the basic concepts of the electrical science and related physics-based relationships, and introduces a number of mathematical formulas to understand the processes mentioned above. To dive smoothly into electrical engineering, one must fully understand the basic terms and concepts: voltage, amperage, resistance, frequency and power. This text is focused on the last one.
Active, reactive and apparent power
All of us have likely heard about electric power. When we choose electrical household appliances, e.g. refrigerators, ovens, and kettles, we look at many parameters, including power, expressed in watts or kilowatts.
Power measures the work that a physical system can do in a unit of time. The following formula describes that relation:
Key:
W – work [J];
t – time [s]
We distinguish three types of power, combined into the so-called "power triangle":
- Active power;
- Reactive power;
- Apparent power.
According to mathematical interpretation, the vector sum of active power (P) and reactive power (Q) equals apparent power (S).
Active power
Also known as "true or real power" or "useful power", active power is generated during transforming work in the devices into heat, mechanic or light energy. The unit of active power is a watt, and its mathematical interpretation is as below:
Key:
P – active power [W];
U – voltage [V];
I – amperage [A];
cosϕ – phase factor is the ratio of active power to apparent power (and related to the phase shift between the current and voltage).
Active power is defined as the average value of instantaneous power.
It is measured with a wattmeter. Let us also mention the Aron circuit, which measures active power in a three-wire three-phase system.
Reactive power
It is also called "useless" or "watt-less" power and does not participate in using electric power to provide the receiver with energy. However, it does participate in storing the energy of magnetic and electric fields. Reactive power is a necessary component of power for the proper operation of electric equipment, such as transformers or electric motors.
Reactive power is expressed as the product of effective voltage and effective current and the sine of the phase shift between the current and voltage.
Key:
Q – Reactive power [VAR];
U – voltage [V];
I – amperage (current) [A];
sinϕ– sine of the phase shift between the effective current and voltage.
There are two types of reactive power:
- Capacitive reactive power Qc, which characterises capacitors or long electric wires;
- Inductive reactive power QL, which characterises all devices with wire coils, e.g. electric motors or transformers.
Apparent power
Just like active and reactive power, apparent power is the product of voltage and amperage. As it is a vector sum of the two power component parts, we sometimes call it "total power".
Key:
S – apparent power [VA];
U – voltage [V];
I – amperage [A]
Reactive power compensation
Delivering electric power from a power plant to end users involves its transmission over very long distances. As mentioned, this generates capacitive reactive power. It is vital to reduce the reactive power received, because it causes significant transfer energy losses, and this requires using electrical cords with bigger diameters of the wires. Another issue is that reactive power reduces the voltage in transformers and transmission power lines. To counteract those adverse effects of reactive power, end users (especially those consuming lots of energy) compensate for reactive power to lower their energy bills. To reduce inductive reactive power, one can, for example, connect batteries of capacitors, and to reduce capacitive reactive power, one can use batteries of inductors.
Reducing reactive power is beneficial both for the facility owner, as they pay lower power bills, and for the power distributor, for whom the reduction in transmission losses means savings on the highly costly replacement of transmission lines and adjustment of transformer networks.
This is just basic information related to electric power, considering the mathematical relationships and definitions of the different components of total power. An important issue is a need for energy consumers to reduce reactive power. To fully understand the meaning of the concepts described here, we would have to analyse the physical phenomena that occur during the successive stages of electricity generation, transmission and consumption in electrical appliances.
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