Metering Energy Quality Indicators (MicroLogic E)

Presentation

The MicroLogic E trip unit provides, using the communication network, the measurements and quality indicators required for energy management:

Reactive power measurement
Power factor PF
cos ϕ
Total harmonic distortion THD
Distortion power measurement

The energy quality indicators consider:

Reactive energy management (cos ϕ metering) to optimize the size of the equipment or avoid peak tariffs.
Management of harmonics to avoid degradation and malfunctions during operation.

Use these measurements and indicators to implement corrective actions to maintain energy quality.

Current THD

The current THD is defined by standard IEC/EN 61000-2-2.

The current THD is a percentage of the rms value of harmonic currents greater than 1 in relation to the rms value of the fundamental current (order 1). The MicroLogic E trip unit calculates the total harmonic current distortion THD up to the fifteenth harmonic:

The current THD can be higher than 100%.

Use the total harmonic distortion THD(I) to assess the deformation of the current wave with a single number. The following table shows the THD limit values.

THD(I) value	Comments
THD(I) < 10%	Low harmonic currents: little risk of malfunctions.
10% < THD(I) < 50%	Significant harmonic currents: risk of heat rise, oversizing of supplies.
50% < THD(I)	High harmonic currents: the risks of malfunction, degradation, and dangerous heat rise are almost certain unless the installation is calculated and sized with this restriction in mind.

Deformation of the current wave created by a nonlinear device with a high THD(I) can lead to deformation of the voltage wave, depending on the level of distortion and the source impedance. This deformation of the voltage wave affects all of the devices powered by the supply. Sensitive devices on the system can therefore be affected. A device with a high THD(I) may not be affected itself but could cause malfunctions on other, more sensitive devices on the system.

NOTE: THD(I) metering is an effective way of determining the potential for problems from the devices on electrical networks.

Voltage THD

The voltage THD is defined by standard IEC/EN 61000-2-2.

The voltage THD is the percentage the rms value of harmonic voltages greater than 1 in relation to the rms value of the fundamental voltage (first order). The MicroLogic E trip unit calculates the voltage THD up to the fifteenth harmonic:

This factor can in theory be higher than 100% but is in practice rarely higher than 15%.

Use the total harmonic distortion THD(V) to assess the deformation of the voltage wave with a single number. The following limit values are commonly evaluated by energy distribution companies:

THD(V) value	Comments
THD(V) < 5%	Insignificant deformation of the voltage wave: little risk of malfunctions.
5% < THD(V) < 8%	Significant deformation of the voltage wave: risk of heat rise and malfunctions.
8% < THD(V)	Significant deformation of the voltage wave: there is a high risk of malfunction unless the installation is calculated and sized based on this deformation.

Deformation of the voltage wave affects all devices powered by the supply.

NOTE: Use the THD(V) indication to assess the risks of disturbance of sensitive devices supplied with power.

Distortion Power D

When harmonic pollution is present, calculation of the total apparent power involves 3 terms:

The distortion power D qualifies the energy loss due to the presence of harmonic pollution.