Cihan Şenel  
Technical Sales Engineer  
Aktif Mühendislik

Nowadays, it is seen that during the projecting of the plant in many different enterprises, the projects which give wrong results with standard applications are drawn regardless of the load characteristics and the speed of change of reactive power requirement. The selection of the switching element in the compensation systems is most important thing for correct compensation. This article is aimed at helping decision makers and project designers to decide which type of switch is right for the facility concerned.


Low - voltage compensation systems are called systems with phase - to - phase voltage below 1 kV. These systems can be switched by contactor or thyristor module. Choosing the right product; minimizing to initial investment cost, operating cost and reactive penalty costs.

II. In which situations/switching types should be preferred to switching by contactor?

Contactor switching is the lowest solution in terms of initial investment cost. It is the best solution for load types and installations where reactive load exchange is stable. The contactor is a mechanical element that activates the step within 1-1.5 seconds after the trigger signal from the reactive power control relay. The longer the reactive power requirement is, the more accurate it is to use the contactor. The change in reactant charge requirement should not be shorter than the discharge period. The step will be deactivated after the reactive power request ends, the discharge time of the low voltage condenser is completed in approximately 40 seconds. Since this stage cannot be re-activated again, correct compensation will not be possible if there is a demand for less than 1 minute.

III. In which situations/switching types should be preferred to switching by thyristor module?

The thyristor is an element that can accommodate the microcontroller in a modulated microcontroller and can switch the capacitor in 1-2 sinus period thanks to the zero transition technology without waiting for the discharge time of the capacitor following the referenced sinus junction. Thyristor modules are used in installations where the number of switching is high and the contactor cannot meet the requested switching speed.

Is the appropriate choice of switching elements in low voltage compensation systems?

Graph 1 - Reactive load change when 20 crane loads belonging to a port are operated simultaneously

As can be seen from the above example load change chart, this reactive load will not meet the demand properly because a system with contactor will wait for the discharge time. The use of a thyristor module is the most accurate switching method in terms of making the compensation at this point. The thyristor modules allow the reactive power control relay to transmit the signal from the thyristor trigger output to the thyristor module and the thyristor module to trigger the thyristor module when the most suitable zero crossing point is reached. It is necessary to use a thyristor module in installations where the need for reactive load is constantly changing.

Properties Switching with Contactor Switching with Thyristor Module
Zero crossing
technology with
voltage tracking
No Yes
Minimum time
between two
switching times
1 minute Less than 1 second
Number of
On / Off
Limited by Mechanical Life Unlimited


Considering the plant / system to be installed, it should be selected as contactor for the facilities where the switching needs are not intensive and not short time. For the intensive and short-term facilities, the switching type should be selected to the thyristor module.


[1] TS EN 61439-6 Low-voltage switchgear and controlgear assemblies - Part 6: Busbar trunking systems (busways)