Connection rules for possibly disturbing loads in Low Voltage Networks (IEC 61000 – 3 –11)

The limitation of disturbances caused by fluctuating 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 – flicker & voltage fluctuations

Main sources and examples

Rapid voltage changes in LV networks are mainly caused by fluctuating loads in consumer’s installations, operation of transformer tap changers or other operational adjustments in the system.

Nuisance effects

The main problems are:

• Effects on lighting
• Tripping of protections
• Fluctuating torque
• Malfunction of electronic devices

Limits

A parameter of importance is the network impedance. The relative voltage change associated with a load variation is given by:

with:

U: voltage at the point of connection (Pcc)
Delta P: active power variation
Delta Q: reactive power variation
R: resistance of the network at Pcc
X: reactance of the network at Pcc
At LV, X/R varies between 0,5 and 1 depending on the type of cable or line.

Connection rules applied in Belgium

The basic principle of the connection rules is to find a compromise between the “strength” of the network and the disturbance level caused by consumer’s installations To do this, we need to evaluate the network impedance as defined in IEC 61000-3-11. To continue, the workflow proposed in the figure below is applied.

Figure – Connection assessment of a fluctuating load

To do this, two easy-to-use calculation worksheets have been created with the different variables and calculations to be executed for the assessment.

If the disturbing loads don’t meet those criteria, a supplementary evaluation stage (Stage 3) is considered with other evaluation methods and criteria. Necessary data

The necessary data:

• short-circuit level at the feeding MV/LV substation
• transformer characteristics
• length and type of cables / lines that connect the load to this substation