Verification of voltage drops

Any wiring system flown by a current causes voltage drops between its point of origin and its end. In an electrical installation, the calculation of voltage drops can meet two objectives:

• Either know the actual residual voltage at a point in the installation (distributor or receiver)
• Or check that the requirements of the standard are met (article 525 and table G.52.1 of IEC 60364-5-52).

Voltage drops in consumers’ installations

Figure 1: Extract from table G.52.1 of standard IEC 60364-5-52

In the second case, it is necessary to determine what the standard calls the origin of the installation: if the installation is supplied from a LV source, the answer is clear and the results corresponding to the two objectives are identical; on the other hand, if the installation is more complex with, for example, HV / LV distribution transformers, the source of the voltage drop can be considered to be the secondary terminals of the HV / LV transformer. The results of the two objectives may then differ depending on the presence or absence of a tap-changer on the transformer. Note that, in all cases, the voltage drop in percentage is expressed according to the network nominal voltage.

Calculation of the voltage drop in a wiring system

1) Approximate formula proposed by the standard IEC 60364-5-52

• b: coefficient depending on the type of circuit (three-phase, two-phase or single-phase)
• ρ1: resistivity of conductors
• λ: reactance per unit length of conductors
• cos φ: power factor in the wiring system
• L: length of the wiring system
• S: cross-section of conductors
• IB: operating current

This approximate formula can be applied for balanced three-phase, two-phase or single-phase circuits with a steady-state cos ϕ greater than 0.8.

2) Vector method

The AFNOR FD C 15-500 documentation booklet offers a more accurate calculation based on the vector representation of the voltage and current in the wiring system.

U₁    Line-to-neutral or line-to-line voltage at the origin of the circuit

U₂    Line-to-neutral or line-to-line voltage at the end of the circuit

ϕ     Phase difference angle at the end of the circuit due to the load

Z      Circuit impedance

Vector representation of voltage drop

But again, possible imbalances in three-phase circuits are not considered.

3) Accurate calculation of voltage drop

The accurate calculation of the voltage drops, whatever the imbalance, must therefore consider:

• the operating current flowing in the phase
• the resistance and reactance of the phase conductor
• the operating current flowing in the neutral
• the resistance and reactance of the neutral conductor
• the power factor in the wiring system

The calculation must be done for each phase. Furthermore, the sum of the voltage drops in several successive wiring systems must be vectorial in order to consider any variations in the power factor on these wiring systems.

Finally, if the installation includes motors, the voltage drop must be checked during the start-up phase in addition to the steady state; in this case, the starting current and the associated power factor, generally around 0.35, will be considered.

This method is used in elec calc™.

Controls with elec calc™

elec calc™ allows the calculation of the various voltage drops within the electrical installation:

• Individual voltage drops of a component
• Global voltage drops from source to receiver
• Voltage drops in steady state

Voltage drops during start-up

The voltage drop is calculated from the actual currents flowing in the phases and the neutral, considering the phase difference angles generated by the impedances of the various components.

For a given operating mode, the voltage drop is calculated in steady state and during start-up. Each start-up is considered to be the most unfavorable, therefore with the entire installation supplied.

The user can define specific voltage drop limits for receivers, transformers, inverters and drives in steady state and, if necessary, during start-up. elec calcTM issues an alert if the defined thresholds are exceeded. An alert is also issued if the thresholds defined by the user exceed the thresholds imposed by the normative texts when they exist.

Figure 2: Voltage drop control in elec calc™

Case of transformers – Consideration of tap-changers in elec calc™

A tap-changer is a device that may be present on power transformers to allow the transformation ratio to be adjusted according to the actual voltage level upstream or according to the downstream load.

elec calc ™ allows you to simulate the 3 scenarios:

This setting will only have an impact on the calculation of the downstream voltage drop.

• Case 1 – Without tap-changer: We will add the voltage drop of the upstream network, the internal voltage drops of the transformer and the voltage drop of the downstream network, which will make it possible to obtain the exact residual voltage at each receiver.
• Case 2 – Off-load tap-changer: This device eliminates the difference between the actual voltage of the upstream network and the rated primary voltage of the transformer. This adjustment is generally made when the transformer is commissioned to obtain the rated no-load voltage at the transformer secondary winding. elec calc™ will only consider the internal voltage drop of the transformer and the voltage drop of the downstream network, the initial voltage being the rated no-load voltage of the transformer secondary winding.
• Case 3 – On-load tap-changer: declaring an on-load tap-changerwill cancel the internal voltage drop of the transformer whatever the downstream load. elec calc™ will only consider the voltage drop of the downstream network, the initial voltage being the nominal voltage of the network. This is the option that should be chosen if we are only concerned with the normative verification of voltage drops.

Written by Jérôme Mullie, Calculation Product Manager.

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