More power with less copper

The table below summarizes what type of reactive power prevails in which type of installation. This table is indicative and gives an average of at least 250 practical measurements. If you want to know for sure what the behavior of your installation is, we recommend performing a power quality measurement (or having it performed). Read more about (temporary) power quality measurements.

The general trend is that through the use of electronics and electronic drives:

• inductive reactive power decreases;
• capacitive reactive power increases;
• harmonic reactive power increases.

Fines and claims

A too high reactive power can lead to a fine from the network operator. Too much reactive power leads to extra burdensome flows and therefore to extra load for the network operator. With harmonic reactive power, the network operator can file a claim if limit values ​​are exceeded.

Installation overload

We need to add the actual power (kW) and reactive power (kVar) together to arrive at the total power (kVA) that must be able to run through the installation. In comparison with the beer glass model, this means that we add the beer and foam together. To prevent flooding of the beer glass, a larger beer glass must be invested. In practice, a higher connection capacity at the network operator, more transformer capacity and a heavier installation will be required.

A higher energy bill due to extra losses

More reactive power also means more power through the installation. More current through the installation means higher losses due to the impedance (resistance) of the installation. We also call this the watt losses. This in turn ensures that more kWs are included (P = I2 * R). These losses, converted into a comparison with beer glass, mean that - no matter how strange it may sound - there is more beer in the glass that is not used effectively, but is absorbed by heat in, for example, cables and transformers.

Derating of transformers and generators

Harmonic reactive power ensures a smaller available capacity of transformers and generators. Due to the higher frequencies of the harmonics, additional copper and eddy current losses and extra losses are generated by inductive and capacitive couplings. Back to the comparison with the beer glass: due to harmonic currents, the beer glass cannot be completely filled.  

Various publications - including the application guides of the IEEE519 - publish derating curves. Based on these publications and our own measurement experiences, we use the following rough guideline for derating transformers and generators. 

Faltering generators

Generator controls encounter problems with capacitive currents. The output voltage then becomes unstable. The capacitive reactive power must, in any case, be less than twenty percent of the nominal generator power. For a stable generator operation, however, we recommend avoiding the capacitive reactive power as a whole.

So there is every reason to reduce unwanted reactive power. There are three basic solutions for reducing reactive power:

• Cos-phi bank (capacitor bank)
• Static VAR generator (electronic cos-phi improvement)
• Active dynamic filter / harmonic filter

The capacitor bank is the most well-known solution for reducing reactive power and has been used for decades. The capacitor bank is - as the name implies - a cabinet full of capacitors with which the reactive power for the coil is supplied. As a result, the reactive power for the capacitor bank has disappeared and a cos-phi of 1 is measured.

For example, if the load absorbs 50kVar of inductive reactive power, and we then put a capacitor bank with a capacity of 50kVar, the complete induction is canceled and the cos-phi becomes 1 through the transformer.

All the inductive reactive power is supplied by the capacitor bank. This relieves the main distributor, transformer and customer connection, creating extra power space.

Advantages of capacitor bank:

• Relatively cheap
• Simple technology
• Modular

Disadvantages of capacitor bank:

• Relatively slow due to relay switching time
• Overloaded by harmonic currents
• Does not adapt to changes in the network
• An outdated capacitor can cause an unwanted upset harmonic 

A capacitor bank cannot be used:

• in a network with capacitive capacity, with the risk of resonances and fire;
• in an installation with a lot of harmonic power, the capacitor is a short circuit at higher frequencies;
• in installations where connected equipment can change;
• with rapidly changing loads and dynamic processes.

A static VAR generator (SVG) is a fast electronic and stepless reactive current compensation without capacitors. The SVG measures the inductive current [1] and injects a current that is phase-shifted with the voltage [2] such that the current for the filter [3] is neatly in phase with the voltage.

Working principle of a static VAR generator

SVG advantages over a capacitor bank:

• Compensate both inductive and capacitive reactive power
• Superfast response time (<20ms)
• Fast and stepless, which means that over- or under-compensation is not possible
• More compact, lower in weight
• Less energy loss
• Not overloadable and insensitive to resonance and interharmonics
• Maintenance-free

Over- or under-compensation is not possible with a Static VAR Generator

Disadvantage Static VAR Generator (SVG):

• Higher purchase price than a capacitor bank

Typical areas of application of an SVG are:

• In installations with rapidly changing loads where a capacitor battery is too slow
• In capacitive networks where the emergency power generator does not turn up properly
• In installations with equal-load phases where more power space is required

An active harmonic filter (AHF) filters harmonic currents with an operating principle that is based on the principle of noise cancelling. The filter measures the current contamination [1] and injects a counter-current [2] that is of such a shape that the current for the filter [3] becomes nicely sinusoidal again. This compensates for all the harmonic reactive power. The harmonic filter is also capable of compensating both inductive and capacitive reactive power, with which such a filter can be used universally and is not overloaded.

Working principle of an active filter

All the harmonic reactive power is supplied by the active filter. This relieves the main distributor, transformer and customer connection, creating extra power space.

Advantages of an active harmonic filter:

• Compensate for both inductive, capacitive and harmonic reactive power
• Even more power space by limiting derating power transformer
• Super-fast response time (<5ms) and continuous adaptation to the load
• All power quality problems can be solved
• Fewer losses, lower footprint
• Not over-loadable and insensitive to resonance and interharmonic

Disadvantage of active harmonic filter:

• Higher purchase price than capacitor bank

The question is often asked whether central or decentralized compensation is required. The size of the required compensation capacities, the structure of the installation and the installation options on-site determine the best solution.

The total reactive power flows through the entire installation without compensation. The sub-distributors, main distributor, main protection and the transformer are charged with reactive power.

Decentralized compensation

With decentralized compensation, the compensation system is placed with the consumers.

Advantage of decentralized compensation:

• The entire installation is relieved of the reactive current

Disadvantages of decentralized compensation:

• Higher purchase and maintenance costs than with central compensation
• Relatively high installation costs because the compensation system is placed at the sub-distributor

Central compensate

With central compensation, the compensation system is often placed on the main distributor of the installation (directly behind the transformer). The main protection, the transformer, and the main connection are thus relieved of the reactive power. The reactive power remains in the internal cabling and sub-distributors.

Benefits of central compensation:

• Lower purchase and maintenance costs
• Relatively low installation costs in the room where the low-voltage main distributor is located
• The most important installation components are relieved of reactive current

The disadvantage of central compensation:

• The sub-distributors and the cabling from and to the sub-distributors is still charged with the reactive current

The apparent power is the sum of the active power (effective power) and reactive power (ineffective power). Reactive power is not converted into useful energy and can consist of inductive, capacitive and harmonic reactive power or a combination of these. The use of electronics reduces inductive reactive power and increases harmonic and capacitive reactive power. As the reactive power increases, the apparent power increases and thus the required capacity of the installation. Harmonic reactive power also ensures a smaller available capacity due to the derating of transformers and generators.

Inductive reactive power can be reduced by applying a capacitor bank. With an active dynamic filter, all types of reactive power can be reduced and the derating of the transformer or generator is limited. This cuts the knife on two sides.

Continuous measurement and monitoring is necessary to determine the correct compensation system and to monitor it.

Reducing reactive power leads to:

• Less power through the installation, transformer and grid connection
• Fewer watt losses, so a lower energy bill
• A larger available transformer or generator capacity if an active filter is used

The compensation to be applied depends on the type of power that occurs in the installation.

Deliberately come to the right solution

Simply installing capacitor banks or active filters seems easy but is not cost-effective and can even be dangerous. Placing capacitor banks in installations with too many harmonics can lead to resonances due to the interaction of the harmonic currents and the capacitors in the reactive current compensation. In addition, it can lead to overloading of the capacitor bank with all its consequences. Placing an active filter if it is actually not necessary or if the intended effect is not forthcoming, is a waste of money.

That is why it is important to first perform a measurement - preferably for a longer period of time - with which the correct compensation technique and capacity can be determined. We always go through a ten-step plan to place a compensation system at fortop.