Table of contents
Reactive Power at a Glance
Reactive power is electrical power that oscillates between the source and the load in an AC grid without directly performing useful work. It is mainly caused by inductive and capacitive loads and affects the loading of cables, transformers and other electrical assets. Power factor correction reduces the amount of reactive power circulating in the grid and can improve efficiency, power factor and grid stability. Reliable assessment requires suitable measurement methods and power quality analysers.
Key Takeaways
- Reactive power is specified in volt-amperes reactive (VAR).
- It does not perform mechanical work, but it is relevant for the operation of many electrical loads.
- Active power, apparent power and reactive power can be explained using the power triangle.
- Power factor correction can reduce grid losses and improve the power factor.
- Reactive power measurement depends on the grid system, load structure and selected calculation method.
- Power quality analysers can evaluate different types of Q separately.
What Is Reactive Power and Power Factor Correction?
Official definition: According to VDE standard 0100-710, reactive power refers to the electrical power that flows back and forth between the phase conductors and the neutral conductor of a three-phase network but is not capable of performing mechanical work. It is measured in volt-ampere-reactive (VAR).
Power factor correction is the process of balancing the reactive component present in the grid to increase the efficiency and stability of the grid. This is often done with the help of capacitors that generate opposing reactive power and thus reduce the total amount in the grid. This compensation is important to improve the power factor of the grid, resulting in less of this component circulating in the grid. The power triangle is a concept that illustrates the relationship between active power, apparent power and reactive power.
Example: Let's assume an industrial company operates several large electric motors that generate considerable inductive reactive power. Without compensation, the energy supplier would have to provide this additional component, which would lead to increased grid losses and lower efficiency. By using capacitors for compensation, the company can generate its own compensating component and thus reduce the load on the grid.
Reactive power compensation offers a variety of benefits, including improving energy efficiency, reducing energy costs and increasing grid stability. In many countries, certain regulations on compensation are mandatory to ensure the quality and stability of the electricity grid.
Overall, reactive power and its compensation play a crucial role in modern energy supply and are of great importance for companies and energy supply companies to reduce unnecessary reactive current and make the grid efficient.
What Is the Difference to Active Power/Apparent Power?
The terms active power, apparent power and reactive power are fundamental concepts in electrical engineering that describe the relationship between different types of electrical power.
| Type of power | Unit | Meaning | Technical context |
|---|---|---|---|
| Active power (P) | Watt (W) | Usable electrical power that performs work. | For example for motors, lighting or electrical devices. |
| Apparent power (S) | Volt-ampere (VA) | Total electrical power present in the grid. | Composed of active power and Q. |
| Reactive power (Q) | Volt-amperes reactive (VAR) | Electrical power that oscillates in the grid and does not perform mechanical work. | Mainly caused by inductive and capacitive loads. |
Active Power (P):
Active power is measured in watts (W).
It represents the actual electrical power used by a device or system in a power grid to do work. In other words, it is the useful power that drives a motor, generates light or operates electrical appliances, for example.
Apparent Power (S):
Apparent power is measured in volt-amperes (VA).
It is the total electrical power present in a power grid, regardless of whether it is actually used to perform tasks. Apparent power is made up of active power and Q and can be regarded as "apparent power".
Reactive Power (Q):
This quantity is also measured in volt-ampere-reactive (VAR).
It represents the electrical power that flows back and forth between the phase conductors and the neutral conductor of a three-phase network, but does not perform any mechanical work. It is primarily generated by inductive and capacitive loads and is required for the operation of these loads, but does not contribute to the performance of tasks.
In a power triangle, which visually represents the relationships between active power, apparent power and reactive power, the active power corresponds to the horizontal side of the triangle, the apparent power is the hypotenuse of the triangle, and Q is the vertical side of the triangle.
An active power meter is an instrument that measures the actual active power in an electrical circuit. It helps to monitor and control energy consumption.
To summarize: Active power is the actual, usable power, apparent power is the total power in the grid, and Q is the power that is not used to perform tasks but is necessary for the operation of inductive and capacitive loads. These concepts are important to understand and optimize the efficiency and stability of power grids. An active power meter is a useful tool for monitoring energy consumption.
You can gain a deeper insight into the calculation and interaction of active, apparent and reactive power in our Info-Letter No.25 "Definition of power measurement parameters according to the DIN 40110-2 and IEEE 1459 standards":
INFO-LETTER No. 22
Definition of measured power values according to the DIN 40110-2 and IEEE 1459 standards
How Is Q Calculated?
This electrical quantity is calculated using the following formula:
Reactive power (Q) = apparent power (S) × sin(φ) Q: Reactive power in volt-amperes-reactive (VAR). S: Apparent power in volt-amperes (VA). φ: Phase shift angle between active power (P) and apparent power (S).
The phase shift angle (φ) is crucial for calculating the amount of Q. It can be positive or negative, depending on whether inductive or capacitive loads are involved.
For inductive loads (e.g. electric motors), the phase shift angle is positive, as Q lags behind the active power. In this case, the formula is
Q = S × sin(φ)
For capacitive loads (e.g. capacitors), the phase shift angle is negative, as Q leads the active power. In this case, the formula is
Q = -S × sin(φ)
Power factor correction attempts to minimize the phase shift angle (φ) in order to reduce Q. This can be achieved through the targeted use of capacitors or other compensation devices.
In addition to the formula for calculating Q, there are reactive power compensation calculators that can be used in more complex systems and networks to determine the exact amount of compensation required.
The unit of Q is Volt-Ampere-Reactive (VAR), which indicates the amount of electrical power required in a system for the formation of electromagnetic fields in inductive devices or to compensate for capacitive loads. This is an important aspect in electrical engineering to ensure the efficiency and stability of power grids.
How Is Q Measured?
The measurement of reactive power in multi-conductor circuits is defined in the standards DIN 40110-2 (Germany) and IEEE 1459 (International). The calculation methods of these standards therefore form the basis for power calculations for modern measuring devices. All devices in the A. Eberle product range - from permanently installed devices to mobile network analyzers - use the DIN 40110-2 calculation method.
Measuring reactive power is an important step in evaluating the condition and efficiency of an electrical system. Various methods and instruments are available for this measurement. Here are some important aspects of the measurement:
Wattmeter
A wattmeter is an electrical measuring device used to determine the active power (in watts) in an electrical system. Active power can be measured by inserting a wattmeter into the circuit. To calculate Q, the formula for calculating the reactive power can be used:
Reactive Power (Q) = √(S^2 - P^2), with:
- Q: Reactive power in volt-amperes-reactive (VAR).
- S: Apparent power in volt-amperes (VA).
- P: Active power in watts (W).
Oscilloscope
When assessing Q with an oscilloscope, voltage and current waveforms are analyzed. The phase shift between voltage and current can be read on the oscilloscope. A phase shift angle that is not zero indicates the presence of reactive power. The amount of Q can be calculated using trigonometry.
Three-Phase Current Measurement
In three-phase systems, special three-phase wattmeters or three-phase power meters are used to measure active power and apparent power in the three phases. The measurement of Q in three-phase systems usually requires more complex instruments and calculations, as the phase shift between the phases must be taken into account.
Circuit Methods
In some cases, special circuits are used to measure this quantity. For example, this measurement can be performed with compensation capacitors to determine the amount of compensation required. Here, capacitors are added or removed to minimize the phase shift angle and thus compensate for Q.
It is important to select the correct measurement instruments and procedures depending on the specific requirements of the system. This measurement is critical to ensure that electrical systems are operating efficiently and stably and to determine the need for power factor correction.
Measuring Q With Power Quality Analyzers From A. Eberle
With our power quality device family, consisting of mobile network analyzers and permanently installed power quality measuring devices, you have the unique option of subdividing and outputting Q in various independent measured variables:
- Fundamental oscillation displacement reactive power
- Harmonic distortion reactive power
- Distortion reactive power
- Modulation reactive power
- Unbalance reactive power
Significant cost reductions can be achieved, particularly when evaluating the use of filter technologies, such as harmonic filters to reduce distortion components. This can also have a positive effect on grid charges. For power quality measurement methods, IEC 61000-4-30 provides an important neutral reference point. At interconnection points between the industrial grid and the public grid supply, we therefore recommend the use of a permanently installed power quality/grid analysis solution such as our permanently installed power quality analyzer with fault recorder function "PQI-DE". The measuring device can store the various types both locally on site and transmit them remotely to various interfaces (see illustration).
Performance Measurement in Practice
Our mobile power quality analyzers (PQ-Box 150, PQ-Box 200 and PQ-Box 300) offer a wide range of setting options to correctly measure the different types of power depending on the application.
Selection of Calculation Methods According to Application
While displacement and distortion components are generally meaningful at every measuring point, the boundary conditions of the measurement and the position of the measuring point in the grid must be taken into account when assessing unbalance components in order to obtain meaningful measurement data.
As an example, the following diagram shows a 400 V local network with a local network transformer, predominantly single-phase connected households and an industrial operation with a two-phase load (e.g. welding equipment).
The decisive factor for measuring unbalanced reactive power is therefore primarily the length of the line section that is loaded by the measured value. When measuring in a residential area in which the individual loads are predominantly connected in single phase, very large unbalances occur. However, the measurable unbalance component is not very meaningful here, as the load of the three-phase system changes at each nearby node and therefore no major section of the grid is loaded unbalanced.
In contrast, it makes sense to consider unbalance reactive power when measuring directly at the local network transformer or at long supply lines for unbalanced loads.
The measurement of modulation reactive power is highly dependent on the selected measurement interval. It therefore only makes sense to take this component into account when calculating the collective apparent power if loads with periodically fluctuating power are present and the modulation frequency of the power fluctuation is known
Examples/Concrete Applications of Power Factor Correction
Photovoltaic Systems (PV systems):
- Power factor correction in PV systems with inverters is crucial for minimising the effects of capacitive or inductive reactive power generated by inverters. The inverter can be set in such a way that the corresponding component is reduced to an optimum level.
Industrial Systems With Electric Motors:
- In industrial systems, electric motors often generate inductive reactive power. Power factor correction through the use of capacitors or other compensation devices is important in order to increase energy efficiency and avoid overloads.
Inverters in Energy Technology:
- Power factor correction in inverters is about adjusting the settings so that the generated component is optimized. This helps to minimize grid fluctuations and improve grid quality.
Private Households With Electric Drives:
- Electrical household appliances can generate Q. Power factor correction in private households increases the power factor and optimizes the efficiency of the electricity grid.
Cables and Transmission Lines:
- When transmitting electrical energy over long distances, capacitive reactive power is generated in the cables. Choked compensators can improve the power quality and increase transmission efficiency.
Three-Phase Motors:
- In industrial applications with three-phase motors, power factor correction is important in order to optimize the power factor and minimize grid losses.
Series Connection of Compensators:
- In complex industrial networks, the series connection of compensation devices can be used to compensate for Q and ensure the stability of the system.
Power Factor Correction in Private Households:
- Power factor correction in private households is relevant for optimizing energy consumption and improving the overall efficiency of the electricity grid.
Choked Power Factor Correction:
- Choked compensation is a specific method of minimizing the effects of this quantity in electrical systems.
Power factor correction is of great importance in various applications in order to improve the quality and efficiency of power grids, minimize grid losses and optimize the overall efficiency of electrical systems.
FAQ - Frequently Asked Questions
What is reactive power in simple terms?
Reactive power is electrical power that oscillates between the source and the load in an AC grid without directly performing useful work. It is mainly caused by inductive and capacitive loads.
Why is reactive power important in the electricity grid?
Reactive power affects the loading of cables, transformers and other electrical assets. If too much of it circulates in the grid, grid losses can increase and the overall efficiency of the electricity grid can decrease.
How does reactive power differ from active power?
Active power is actually used to perform work. Reactive power does not directly contribute to useful work, but it is required for the operation of certain electrical loads.
How is reactive power calculated?
Reactive power can be calculated using the relationship between apparent power and the phase shift angle. The article explains this with the formula Q = S × sin(φ).
How is reactive power measured?
Reactive power is determined using suitable measuring devices and calculation methods. In modern measuring devices, standards such as DIN 40110-2 and IEEE 1459 form the basis for power calculations.
When is power factor correction useful?
Power factor correction is useful when inductive or capacitive loads worsen the power factor and cause grid losses, higher loading or efficiency losses. Typical examples include industrial systems, electric motors, inverters, cables and transmission lines.
