Basic solutions to mitigate harmonics

To limit the propagation of harmonics in the distribution network, different solutions are available and should be taken into account particularly when designing a new installation.

Position the non-linear loads upstream in the system

Overall harmonic disturbances increase as the short-circuit power decreases.

All economic considerations aside, it is preferable to connect the non-linear loads as far upstream as possible (see Fig. M24).

Fig. M24 – Non-linear loads positioned as far upstream as possible (recommended layout)

Group the non-linear loads

When preparing the single-line diagram, the non-linear devices should be separated from the others (see Fig. M25). The two groups of devices should be supplied by different sets of busbars.

Fig. M25 – Grouping of non-linear loads and connection as far upstream as possible (recommended layout)

Create separate sources

In attempting to limit harmonics, an additional improvement can be obtained by creating a source via a separate transformer as indicated in the Figure M26.

The disadvantage is the increase in the cost of the installation.

Fig. M26 – Supply of non-linear loads via a separate transformer

Transformers with special connections

Different transformer connections can eliminate certain harmonic orders, as indicated in the examples below:

• A Dyd connection suppresses 5th and 7th harmonics (see Fig. M27)
• A Dy connection suppresses the 3rd harmonic
• A DZ 5 connection suppresses the 5th harmonic

Fig. M27 – A Dyd transformer blocks propagation of the 5^th and 7^th harmonics to the upstream network

Install reactors

When variable-speed drives are supplied, it is possible to smooth the current by installing line reactors. By increasing the impedance of the supply circuit, the harmonic current is limited.

Installation of harmonic suppression reactors on capacitor banks increases the impedance of the reactor/capacitor combination for high-order harmonics.

This avoids resonance and protects the capacitors.

Select the suitable system earthing arrangement

TNC system

In the TNC system, a single conductor (PEN) provides protection in the event of an earth fault and the flow of unbalance currents.

Under steady-state conditions, the harmonic currents flow in the PEN. Because of the PEN impedance, this results in slight differences in potential (a few volts) between devices that can cause electronic equipment to malfunction.

The TNC system must therefore be reserved for the supply of power circuits at the head of the installation and must not be used to supply sensitive loads.

TNS system

This system is recommended if harmonics are present.

The neutral conductor and the protection conductor PE are completely separate and the potential throughout the distribution network is therefore more uniform.