What is cosine phi and reactive power?

One of the most efficient ways to save energy is to increase the cosine phi and reduce reactive power. Reactive power is the power that devices consume but do not convert into motion, heat or light. So this power is lost, so to speak. How exactly does this work? The experts at Sensorfact explain.

Table of contents

    What is reactive power?

    Appliances and machines consume more power than they actually need. Some of the power consumed is actually converted into heat, light or movement. This depends on the function of the device. Another part is lost and, therefore, wasted.

    Active and reactive power

    The part of the power that does actually get consumed is active power (P). The part that is lost is called reactive power or reactive power. This power is not usefully utilised by the device.

    Reactive power is needed to generate a magnetic and electric field. This power is needed for the proper functioning of a device, but is not used for its ultimate function.

    Reducing this reactive power leads to energy savings since when the proportion of active power increases, a device becomes more efficient. Less energy is wasted, so less power is needed for the same operation of the machine.

    What is cosine phi (cos phi)?

    The difference between the active power and the total power consumed (the power factor) is expressed by cosine phi. The cosine phi is a number between 0 and 1. When the cosine phi is 1, 100% of the energy consumed is converted by the machine into motion, light or heat and therefore there is no waste.

    A cosine phi of 0.9 indicates that 90% of the power consumed is converted into active power. 10% of the power is lost and is, therefore, reactive power. The higher the cosine phi is, the more efficient devices are therefore used. Usually it is between 0.6 and 0.9. So this means that 60-90% of the power is used effectively.

    Apparent power and true power

    The difference between the actual, or true power consumed and the apparent power is the reactive power. So the formula is as follows:
    Reactive power + true power = apparent power

    Here, apparent power is the total power consumed by a device. With a cosine phi of 0.7, the true power accounts for 70% of the apparent power. The reactive power is then 30% of the apparent power.

    Beer glass comparison

    To clarify the relationship between apparent power and true power, the comparison is often made with a beer glass. The beer glass symbolises the power system. The beer is the active power, the foam is the reactive power.

    Both too high active power and too high reactive power can cause the beer glass to overflow. At that point, the power network will become overloaded. The more reactive power there is, the more copper, transformer and connection capacity is needed.

    A decrease in reactive power (the foam) can ensure that there is more room for active power (the beer). This allows more power to be used more conveniently, without flooding the glass.

    A bad cosine phi (power factor)

    Many grid operators use a lower limit for the cosine phi. This is because if the cosine phi is too low, the voltage in the supply lines increases. This creates a lot of heat. This can be dangerous, could cause the machine or system to overload and wear on the power network.

    Often, the lower limit for voltages up to 50 kilovolts (kV) is 0.85. For voltages above 50 kilovolts, the lower limit is 0.8. So it is important to achieve the highest cosine phi possible. The grid operator may charge extra if the cosine phi is too low.

    Penalty for too low cosine phi

    If the cosine phi is too low, the grid operator incurs extra costs because it has to transmit reactive power. Some grid operators therefore even apply a lower cos phi limit of 0.9. If it is lower, extra costs are charged.

    Due to companies’ cosine phi being too low, energy operators see the efficiency of their power grid drop. As a result, they can serve fewer companies with the same connections. Larger transformers and more copper is needed.
    This causes the grid operator to lose revenue. For this reason, a penalty is charged. In this way, grid operators hope to encourage companies to take measures if the cos phi becomes too low.

    Apart from the fact that you need extra connection capacity more quickly, a cos phi that is too high also leads to higher power costs. It is therefore advantageous to make sure you keep your cos phi as close to 1 as possible. That way, you can increase your consumption yourself without a new connection nor risk a fine.

    Disadvantages of reactive power/a low cosine phi

    Besides a penalty, there are other disadvantages of a low cos phi or reactive power that is too high, such as:

    • Power costs for power that are not actually used
    • Greater load on installations, which can cause devices to fail. Business processes can then be disrupted and bottlenecks occur
    • Overload and overheating of the connection
    • More quickly running into the maximum capacity of the connection, requiring it to be expanded

    Causes of reactive power

    Reactive power is caused by magnetism in motors and transformers and capacitors in electronic equipment. Induction motors, for example, use only 80-90% of the power usefully. The rest is used to create a magnetic field in the motor. There are several causes of reactive power.

    Phase shift

    Frequently, the cosine phi also drops considerably when too many inductive devices are connected to the same installation. At that point, phase shift occurs. With a cos phi of exactly 1, there is no phase shift. Additionally, apparent power and true power are equal.

    If the phase shift becomes too large, it may need to be compensated. This is done with capacitors in a capacitor coil. This way, the phase shift and cos phi are reduced to an acceptable level.
    Large consumers in particular suffer from phase shift. If there are many cooling units, equipment, machines, or motor controllers, phase shifts are more likely to occur. And, consequently, the power grid becomes overloaded faster.

    Different types of phase shifts

    Depending on the type of devices and other conditions, two different types of phase shift can be identified: inductive reactive power and capacitive reactive power.

    Inductive reactive power

    With inductive reactive power, in order for devices to work, power is needed to magnetise the coils. This power is called inductive reactive power.

    Capacitive reactive power

    Capacitive reactive power occurs mainly within organisations with a lot of electronics, such as data centres and hospitals. Capacitive reactive power arises from capacitive loads.

    Harmonic reactive power

    Besides phase shift, harmonic reactive power can also occur. This is due to devices that create non-linear loads on the power grid. These devices do not use power in a uniform sine wave, but in irregular pulses.
    Examples of these devices include LED lighting, HVAC systems and computers. The pulses generated by these devices cause power to flow back to other parts of the power network. This is called harmonic pollution and causes reactive power.

    So, in total, reactive power consists of three different types of reactive power: inductive, capacitive and harmonic.

    Reduce phase shift with a capacitor bank

    The inductive reactive power can be compensated with a capacitor bank or capacitor battery. A capacitor bank compensates for the phase shift caused by magnetising the coils.

    A capacitor bank provides the power needed to magnetise the coils. So this reactive power no longer needs to be drawn from the mains.

    The advantages of a capacitor bank at a glance:

    • Lower power costs due to lower power loss
    • Avoiding cos phi penalty by grid operator
    • Less overloading of installations, thus longer service life and lower life cycle costs
    • Less overloading of installations, so fewer business process failures
    • CO2 reduction through reduced power consumption
    • More capacity on the same power grid, so more opportunities to expand

    Reduce phase shift with a static VAR generator

    A static VAR generator prevents phase shift by ‘injecting’ power. This restores the power to full equality with the voltage and returns the cos phi to 1. Because the VAR generator injects power at the right moment, it works against both inductive and capacitive reactive power.

    Avoid a cosine phi penalty and improve your energy efficiency with expert energy advice

    Sensorfact’s energy consultants are ready to help you increase your cosine phi. With our software and hardware, saving opportunities are easily identified.

    Our experts happily recommend measures for you that can help you increase your cos phi. Increasing cos phi is one of the many measures Sensorfact’s experts can consult you on.

    Our customers save up to 15% on their energy bills. With the current gas and electricity prices, this can add up to annual savings of more than €100,000.
    With concrete and targeted advice, we make it easy for you to implement energy savings.

    Are you curious about our product? Then request a demo today, free of any obligations.