Connection rules for possibly disturbing loads in Low Voltage Networks (IEC 61000 – 3 – 12)

Introduction

The limitation of disturbances caused by distorting loads of medium size, at Low Voltage (LV), concerning appliances with input current > 16 A and  75 A is the subject of this article. Now, why did we develop a practical tool in order to easily assess the disturbances? It's simple: the standards dealing with these issues are not exactly easy in use.

Low-voltage disturbances - harmonics
 

Main sources and examples

Harmonics are mainly caused by non-linear loads. This non-linearity can be intrinsic but can also be the result of repeated commutations of power electronic components.

Nuisance effects

One distinguishes 2 kinds of effects:

• Immediate effects: Disturbance of controllers of electronic systems, ripple control receptors, relays and dimmers.
   
• Long-term effects: Heating of capacitors, cables, equipment, transformers and rotating machines; provocation of noise and vibrations
   

Limits

IEC 61000-3-12 proposes an assessment procedure in three stages, with individual currents emission limits, essentially depending upon the short-circuit ratio at the connection point (here for a balanced three-phase equipment):

with:

Ssc = the short-circuit power
Sequ = rated apparent power of the equipment.
Assumption: ZLV is mainly inductive, excluding any resonance

Connection rules applied in belgium

The basic principle of the connection rules is to find an acceptable compromise between the “strength” of the network and the harmonics level caused by consumer’s installations. The key element in finding the compromise is the evaluation of the short-circuit ratio as defined in IEC 61000-3-12.

Having analysed the consumer’s installation and located the major sources of harmonics, the workflow proposed in the figure below is applied.

Figure - Connection assessment of a distorting load

Necessary data

The data needed are:

• Grid parameters to determine the Rsce at the Pcc
   
• Characteristic harmonic currents
   

These data can be determined from measurements on similar loads if necessary.