Line chokes

The supply network may be endangered to the influence of non-linear receivers causing the deformations of sinusoidal voltage run, and thus the increase of losses and disturbances in the operation of other machines and devices connected to that network.
The line chokes are applied in order to limit the harmonic propagations in local NN networks. Moreover they perform other tasks, such like suppressing the commutation overvoltages and in case of a short-circuit they reduce the value of set short-circuit current and the theepness of its accruing. 

Basic functions of line chokes in converters supply systems.
The systems of low-power thyristor converters may be supplied directly from network, without individual transformer. In such circumstances it is necessary to use the line chokes (of ED1N or ED3N type) on the line between the supply network and converter.
These chokes protects both the converter and the supply network.
 


Simplified diagram of converter for supplying the squirrel-cage motor

Controlled rectifiers and frequency converters generate a series of harmonics in the network. These harmonics deform strongly the run of voltage sinusoid, which results in the increase of power losses in all machines and devices supplied from the network.
The line chokes (ED1N or ED3N) limit the propagation of higher harmonics in the network and they suppress the commutation overvoltages arising during thyristors switching over. The application of line chokes causes the weakening of mutual interference of converters during commutation.
The thyristors of converters system often need a protection restraining the accretion of conduction current until the moment when the pnpn structure is switched over to the conduction state. Installation of line chokes is the simplest method to achieve this purpose.
While selecting a choke an attention should be paid to the mutual dependency between the supply network inductance (LS) and choke inductance (LED3N), which should meet the condition (1).

(1)

where:  UTm – value of blocking voltage possible it a given system at the moment before thyristor switching over; 
(diT/dt)crit – critical steepness of thyristor conduction current accretion;  LS – substitute inductance of network and source.

If it results from formula (1) that  LED3N ≤ 0 then it means that there is no need to install the line chokes, because the network inductance limits sufficiently the value of current derivative.
More practical method for determination the technical parameters of line chokes consists in assumption of allowed voltage drop on the choke which should not exceed several per cents of network rated voltage. Knowing the value of load current we can determine, with using of equation (2), the choke inductance assuming a several per cent voltage drop on the choke.

(2)
where: I – rated current of load, f – network voltage frequency, LED3N – line choke inductance.

The characteristics of magnetic way should allow the line choke to enter into the saturation state within the entire range of expected currents of receiver.


ELHAND line choke, type ED3N-0,77mH/70A

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